I wouldn't really say his voice sounds 'better' as such. Comparing In Utero to Nevermind is an impossibility, their aims are vastly different, one is a clear (and excellently produced) pop album and the other more artistically driven.
His voice certainly sounds more natural on In Utero, and this suits the album perfectly, whilst on Nevermind it's more cutting and polished, again suiting the aims of the project.
I find the point about the main goal being to create an accurate interpretation of a bands true sound interesting, in a way i do agree but i've always considered production an art in itself that can really bring a bands songs up to their full potential for the specific aims of the recording, but i suppose this depends on a number of things in reality.