Rashi explains that Abaya explains the Machlokes between R'Yehudah and R' Shimon concerning if the Tzitz is Mesratza if it is not on the Kohain Gadols head. My question is , how then does Abaye (according to Rashi) explain the braysa when it records what R' Yehuda answers R' Shimon proof from Yom Kippur when the Tzizt is not worn. To which R'Yehudah says Tumah Hutrah. We see from this that they are not arguing about the placing of the Tzitz, but if Tuma is hutrah or nidcha.
Whether or not the Tzitz is Meratzeh when it is not being worn is Taluy on whether or Tum'ah is Hutrah or Dechuyah. Rebbi Yehudah says that the Tzitz is not Meratzeh when it is not being worn. If so, how is there Ritzuy on Yom Kipur when the Kohen Gadol does not wear the Tzitz? It must be that he holds that Tum'ah is Hutrah (and thus Ritzuy is not necessay) -- and this is what he indeed says in the Beraisa.
According to Rebbi Shimon, the Tzitz is Meratzeh when it is not being worn, and thus there is Ritzuy, which implies that there is a need for Ritzuy. It must be, then, that Tum'ah is only Dechuyah, not Hutrah.
Therefore, Abaye's explanation of the argument between Rebbi Shimon and Rebbi Yehudah concerning whether the Tzitz is Meratzeh when it is not being worn is dependent on the argument whether Tum'ah is Hutrah or Dechuyah.