1) HALACHAH: MEZUZOS ON HOUSES IN YERUSHALAYIM
OPINIONS: The Gemara says that according to the Tana Kama, houses in Yerushalayim do not become Tamei with Nega'im. The Torah says that only houses built on "Eretz Achuzaschem" -- "land in your possession" (Vayikra 14:34) -- can become Tamei. Yerushalayim is not considered "Achuzaschem," "your possession," because it was not divided among the tribes.
Does the Gemara's comparison of the laws of Nega'im to the laws of Mezuzos imply that the Tana Kama maintains that houses in Yerushalayim are exempt mid'Oraisa from the Mitzvah of Mezuzah?
(a) The RAMBAM (Hilchos Mezuzah 6:6) writes that a synagogue is exempt from the Mitzvah of Mezuzah because of its status as a holy place, just as Har ha'Bayis, the Lishkos, and other sanctified places are exempt (11b). With regard to Nega'im, the Gemara (11b) says that a synagogue does not become Tamei with Tum'as Nega'im because it has no designated owner. Its status as a holy place is not the reason for its exclusion from the laws of Nega'im.
According to the Rambam, why does the Gemara say that a synagogue of a small town ("Beis ha'Keneses Shel Kefarim") requires a Mezuzah? It should be exempt because it is a holy place! The Rambam explains that it was the common practice in small towns to use the synagogues as guest houses. The synagogues were built with intent that they not be sanctified as holy places so that guests would be able to sleep there. (The RITVA (11a) offers a similar explanation.)
The Rambam clearly maintains that one set of criteria determines whether the Mitzvah of Mezuzah applies to a place, and another set of criteria determines whether the laws of Nega'im apply to a place. Accordingly, houses in Yerushalayim require Mezuzos, even though the laws of Nega'im do not apply to them.
(b) RASHI (DH d'Kefarim) writes that a synagogue of a small town requires a Mezuzah because it has "Ba'alim Nikarim" -- its owners are known. It is like a house owned by partners. In contrast, a synagogue of a large town ("Beis ha'Keneses Shel Kerachim") has no discernible owners.
Rashi does not differentiate between the criteria that determine whether the laws of Nega'im apply to a place and the criteria that determine whether a place requires a Mezuzah. The applicability of both Halachos depends on whether the house has a known, discernible owner. When the owner of the house is not identifiable, it does not require a Mezuzah and it is not subject to the laws of Nega'im.
What is the status of the houses in Yerushalayim with regard to the Mitzvah of Mezuzah? Although the houses in Yerushalayim do not become Tamei with Tum'as Nega'im because the land is not owned by anyone, the houses upon the land do have owners, and thus perhaps they need Mezuzos. However, if the houses have owners, then why does the Beraisa prohibit one from renting out a house in Yerushalayim? Since the house has a private owner, he should be entitled to rent out the right to dwell in the house.
Perhaps the answer is that at the allotment of the land during the times of Yehoshua, a condition was made that whoever builds a house in Yerushalayim and uses the land is not allowed to keep others out. Therefore, one may not take rental money for a house in Yerushalayim. However, the house itself has a private owner and thus it requires a Mezuzah.
2) A MEZUZAH ON A HOUSE OWNED BY PARTNERS
QUESTION: The Gemara concludes that according to the Beraisa, the synagogue of a small town ("Beis Keneses Shel Kefarim") does not become Tamei with Nega'im. Even if the owners are known, the fact that it is owned by a group exempts it from the laws of Tum'as Nega'im. It is like houses built on land that was allotted to a tribe but was not yet divided among the specific families. The houses do not become Tamei with Nega'im because they have no individual owner.
The Gemara earlier, however, seems to contradict this law. The Gemara (11b) quotes a Beraisa which says that a house owned by partners does become Tamei with Tum'as Nega'im. According to the Gemara here, a house owned by partners should not become Tamei with Nega'im. How are the two Beraisos to be reconciled?
(a) The TOSFOS YESHANIM and TOSFOS HA'ROSH explain that the Gemara here assumes that the two Beraisos disagree. According to the Beraisa here, a house owned by partners indeed does not become Tamei with Nega'im and is exempt from a Mezuzah.
(b) The Tosfos Yeshanim suggests further that perhaps the two Beraisos can be reconciled. The Beraisa here agrees that a house owned by partners must have a Mezuzah. The reason for the Beraisa's ruling that houses built on land that has not yet been divided do not become Tamei with Tum'as Nega'im is because that land was never owned by one person, and not merely because that land is presently owned by more than one owner. Similarly, there is a difference between the joint ownership of a synagogue of a small town and the joint ownership of a house owned by partners. In the case of the synagogue, the joint owners are not entitled to partition the synagogue and give each owner his portion. In contrast, a house owned by partners may be divided whenever the partners decide to divide it. Therefore, a synagogue is exempt from a Mezuzah (and the laws of Tum'as Nega'im) while a jointly-owned house is not.
(c) The RITVA and RI HA'LAVAN explain that a house owned by partners is normally exempt from a Mezuzah and does not become Tamei with Tum'as Nega'im, as the Beraisa here teaches. However, when each partner is allotted a designated and distinct area of the house, each section of the house requires a Mezuzah and can become Tamei with Tum'as Nega'im. In such a case, the partners share only a common door at the main entrance to the property. Each owner is obligated to affix a Mezuzah on his section of the house.
HALACHAH: The SHULCHAN ARUCH (YD 286:1) rules that a house owned by partners must have a Mezuzah.
3) BINYAMIN'S REWARD
QUESTION: The Beraisa says that Binyamin was distressed when he learned that his portion of Eretz Yisrael would not contain all parts of the Beis ha'Mikdash and Mizbe'ach. In his portion would be only the Mizbe'ach and the parts of the Beis ha'Mikdash to the west of the Mizbe'ach, while in the portion of Yehudah would be the parts of the Beis ha'Mikdash to the east of the Mizbe'ach and a strip right beneath the eastern (and southern) base of the Mizbe'ach. Binyamin was distressed that he would receive only part of the Mizbe'ach and not the entire Mizbe'ach.
As reward for his strong desire to have all of the parts of the Beis ha'Mikdash in his portion, Binyamin merited "to become the host for the Almighty." RASHI explains that this means that "the Aron ha'Kodesh was placed in his portion."
The MAHARSHA (in Zevachim 53b) asks that if Binyamin saw that his portion of land would contain most of the Mizbe'ach except for one small strip on the eastern side of the Mizbe'ach, then certainly he knew that the Mizbe'ach and the area to the west of it would all be in his portion. Accordingly, he already knew that the Aron ha'Kodesh would be in his portion! What, then, does the Gemara mean when it says that Binyamin was rewarded for his feelings of distress by having the Aron ha'Kodesh in his portion? It was already in his portion!
(a) Hash-m revealed to Binyamin that both he and Yehudah would have parts of the Mizbe'ach in their respective portions. Binyamin, however, did not know exactly how this distribution would be executed. All he knew was that he would share the Mizbe'ach with Yehudah. Since he so strongly desired to have all of the Beis ha'Mikdash in his portion, he merited to receive the area to the side of the Mizbe'ach that contained the Aron ha'Kodesh.
(b) The TORAH TEMIMAH (Devarim 33:12) gives a different explanation for what the Gemara means when it says that Binyamin merited "to become the host for the Almighty."
The Gemara in Zevachim (118b) says that the Shechinah dwelled among the Jewish people in three places: in the Mishkan in Shilo, in Nov and Giv'on, and in the Beis ha'Mikdash in Yerushalayim. All three places were in the portion of Binyamin. Binyamin foresaw that he would have the Aron ha'Kodesh in his portion in the Beis ha'Mikdash, while Yehudah would have in his portion only a strip from the Mizbe'ach. As reward for his distress that the eastern base of the Mizbe'ach would not be in his portion, Binyamin merited that the other places in which the Aron ha'Kodesh would reside would be in his portion as well. (This may be the intention of Rashi here as well.)