Kohan In Demand

The Mishna in Negaim (3:1) says that a kohan must pronounce the nega tamah or tahor.  The Gemorah in Yoma (42a) wants to bring a proof from the fact that a kohan needs to shect the parah adumah that shechita is considered an avodah.  The Gemorah says that shechita is not an avodah and parah adumah requires a kohan just like seeing a nega.  The kohan that sees the nega does not need to be kosher for the avodah, it can be a baal mum or even a katan (see Rambam Ch. 9 of Tumas Tzaaras) and therefore presumably he does not have to wear the priestly garments.   Similarly, one would assume the same thing applies to shecting of the parah adumah (Chazon Eish Parah 8:3.)  However, the Tosfot Yeshanim and Rosh in Yoma say that the kohan that shects the parah adumah must wear the priestly garments.  Why?  The Minchas Avrohom (Torah Kohanim pg. 98) suggests that they must hold that garments are not worn for the sake of doing an avodah but that it is a din in the actions of the parah that it requires priestly garments.  That would explain why the Rambam holds even if the kohan gadol does the avodat haparah he still wears the priestly garments of a regular kohan (see Laws of the Parah 1:12 and Mishne L'melech.)  Why does he not wear his regular priesly garments or at least the bigday lavan he wears on Yom Kippur?  It must be the Rambam understood this is a unique law of wearing priestly garments nothing to do with avodah and that is fulfilled with the bigday lavan. 

The Briskor Rav suggests that this need for a kohan extends to the taharah of the metzorah as well.