Well, that is if you can get past the mouthpiece change--while Cs can use the same mouthpieces as the Bb/G instruments, it isn't the best idea to do so.
I've played on both keys, and I honestly have to say that I like Bb better, not only for tuning reasons, but because a lot of music was simply written to be played with a Bb's timbre. However, Gs should still have their place in today's corps. It would be interesting to see a line that includes both Bb and G instruments. For example, think of a trumpet section and a soprano section in the same line. The sopranos could play different music from the trumpets in the same way that many pieces of brass band music contain both trumpet and cornet parts. A sop could act as a link between the mellos and the trumpets (unless one also wants to include flugels as well). Of course, it depends on whether or not the corps wants to play with a mix of timbres, or stick with the raw power of a traditional line . Then again, a line of Bb, F, and G would make warmup exercises a little more difficult...
Either way (sorry in advance), there are solos that don't sound too good on Gs in general. They make great ensemble instruments though!