Depends on the band really. Current band is only a 4 piece singer, guitar, bass and drums and we won't rehearse if someone can't make it.
Last band i was in was a 6 piece band and we would still rehearse if someone couldn't make it. There were a lot of harmony vocals that they would work on or twin guitars we would work on their parts or simply try a few suggested song and run thru them to see how they sounded before adding to set list. For this particular band yes it works ok.
Would have to say that most bands i've been in would normally cancel as it was usually quite rare.
My current band is made up of guys that already play in reasonably successful bands and this was a side project so there are gaps in rehearsals however we just learn new songs at home and all being pretty competent musicians it works out well.
Dave