The "soul" in vocals and guitar comes from, in a large part, from its deviation from the actual beat, usually behind. Bass is 99.9% of the time on the beat, so playing on the beat and yet having that delayed soulful vocal is tough to pull off, while "slurring" the guitar and vocal together is very natural to do (while a bass player is staying on the beat for you so you can roam around it, btw).
A lot of bands with bassist/vocalists have very tight beats and vocals (think Rush). You'll almost never see a variety cover band with only a bass playing lead vocalist (not that they don't exist).
I'm the singer in a variety cover band. Boy is it a bitch. I have to have 100% seperation of mind and fingers. Voodoo chile and stranglehold are tough, but doable.
