Roger (or Davids?) bass parts serve the songs. Guy is a better bass player, of that there is no doubt, but I would say that Rog is a songwriter who happens to play bass.
If you watch the Wall shows, he plays a lot of accoustic guitar and G E Smith plays most of the bass parts. But Pigs and the Animals record in general, is a great piece of work.
Here is a rig run down of the guys involved in the Wall tour, Dave Kilminster, Snowy White and G E Smith.
http://www.youtube.com/watch?v=7s1x-JjZu2I&feature=share&list=PL13D7650D1E3388E9