It would help him avoid getting dominated by characters who can through out long lasting disjointed moves since he currently has no effective means of getting through them. Characters like Pikachu (fair), Kirby (bair), Wario (lot's of moves, especially bite, factoring in his aerial mobility makes it worse), DDD (of all characters right?), ect can just wall him out with high priority sex kicks and there's really not enough lag on them for Sonic to effectively punish, nor do they have enough startup lag to be intercepted. When the opponent decides that they are willing to really just use that one move all match, the fight becomes pretty one sided. Sonic doesn't have major problems with other characters because....they have enough lag for him to get in either after or before the attacks. But he has no method of even touching a character who has a long lasting wall.
A larger hitbox on nair (mostly for the disjontedness) at the BEGINNING would let him have at least have an answer to this situation, even if the hitbox itself only lasted for a short while. I'm not asking to change it into a sex kick to be used as a wall, I'm asking for some method to break aerial walls, as he currently has none. The strong hit of nair really doesn't combo into much either, so it's not going to turn these matchups around completely, but at the very least the opponent will have to use more than a few attacks.
That enough reasons for you? I've said Sonic was good before, but his bad matchups are really just abusing one move against him while he constantly has to run around waiting for the opponent to mess up their spacing on their own, since he has no way of forcing it.