We've had occasions , just like all other bands, where someone cannot rehearse, but we're in the fortunate position of being able to rehearse in our rhyth git's back room, where the PA , all amps, and a drumkit are permanently set up.
Having said that, we've tried reheasring without one of the guys there, and its quite hard work. Its those times when you suddenly realise that you take cues from everyone else, especially the drummer, in our case.
Rehearsals without the singer are just as awkward, for us anyway, and same goes for the both guitarists. The only person the band can rehearse without and still get a somewhat decent practice in, is me