People who brought bikkurim were often ignorant and unable to read the parsha of arami oveid avi themselves, so they avoided performing the mitzvah. The Chachamim at first instituted reading the parsha for those who could not do so themselves, but people still avoided the mitzvah out of embarrassment. Therefore, the Mishna at the end of Bikkurim tells us that the Chachamim instituted that the parsha be read for everyone. Rabeinu Tam writes that this practice of reading the parsha for someone who cannot do so is the same function performed by the ba’al koreh who leins on Shabbos. The Rosh (Megillah perek 3) disagrees with this comparison. By the case of bikkurim, Chazal had to institute a ba’al koreh otherwise people would not perform the mitzvah d’oraysa of bringing bikkurim. When it comes to getting an aliya on Shabbos, there is no mitzvah to get an aliya – argues the Rosh, let those who can read be called up and those who can’t simply not be oleh! Therefore, concludes the Rosh, the oleh should only be someone capable of reading, and the oleh should in fact read along with the ba’al koreh to properly fulfill the mitzvah.
Why according to the Rosh does it seem like the institution of ba’al koreh is only a b’dieved? Just as someone can recite kiddush to be motzi someone else who may or may not be able to read, or someone can recite a bracha and listen to someone else’s tekiyas shofar even if he does not know how to blow tekiyos (assuming the mitzvah is blowing tekiyos), why does it not follow that someone can say a bracha and listen to someone else lein the parsha on his behalf? Floor is open for comments….