I think the reason many of BMW's f-series bikes aren't belt drive now is that many of them are off-road GS models, except for the 'sport-standard' model (I forget it's designation). The sporty types, for some reason, don't dig belt-drive; I think maybe it's because it's easier to change to different sprocket/gearing combinations on chain-driven bikes.
Personally, I'd MUCH prefer a belt-drive. It's interesting that ZERO has gone to belt drive on their streetbikes; this must make them even quieter than before; stealth is always a good thing when you're being a 'hooligan'....