I don't think that the above Rashi indicates that dead people cannot be a "Meilitz Yosher" -- "good advocate" for people in this world. It just means that when people take care of the dead, "she'Aino Metzapeh l'Tashlum Gemul" -- "that he does not look forward for payment in kind." Most people who volunteer in a Chevra Kadisha do not take care of the dead in order that they can Daven at their Kevarim, but rather for the Mitzvah. The contrast is doing favors for live people who should have Hakaras ha'Tov for favors done to them by others. It is therefore normal to anticipate possible payment in kind.

Alternatively, even if one would look forward to Davening at a person's grave when burying him, the Gemara's comments regarding the dead person's advocacy for rain does not seem to be relative to whether or not those praying buried him. It is merely that when the dead are "notified" of the difficulties of Klal Yisrael they will (according to this opinion) Daven for rain. This is unlike doing favors for the living, where the favor is repaid in kind due to the favor that was done.

All the best,

Yaakov Montrose