If you are having a rehearsal, whether it be for Sunday morning service or an outside engagement, should musicians who arent your regular musicians and arent going to be playing for this particular engagement sit in and play during the rehearsals?
Does it do more damage than good, because whatever gets done in rehearsal needs to be what can be reasonably expected during the actual setting? Of course when an outsider tries to do moves/breaks/progressions I try to shut stuff down because that wont be done (now if we're rehearsing just to rehearse, fine, but if rehearsing for something particular coming up, getting everyone accustomed to things that wont be there come gametime is bad). But is the addition in any capacity bad?
If its the case of "our organist or bass player, cant make it so it'll just be our keys and drums" then yeah, they should practice in the rehearsals anyway. But if you NEVER have these instruments, should other musicians at any time sit in and play them during rehearsals?