I always thought that the producer's main job was to make the band sound on the recording like their musical vision. Given that if the band needs tight and technical playing then the best player for that part should be used. If you have a band with two guitarists with very different styles, like early Guns and Roses, you let both do their thing. In my last band the other guitarist was great but was more of a soloist than a tight rhythm player. For the CD I ended up doing all the rhythm tracks, since I wrote them I also knew them better, and the result was far far tighter than with his takes included. It pissed him off initially but since I did his rhythm tracks with my guitars and his rig it didn't totally sound like me playing 4 tracks. His ego-sphincter relaxed and the recording sounded good.