In choir rehearsals, i personally don't like the playtime. In most cases, a band's "playtime" is in the form of over-the-top improv, unexpected progressions, etc. Typically, anything done out of the ordinary from the band will draw peoples attention to them, thus away from the choir director or that choir members part that they are supposed to be focusing in on.
As a director, I don't let one vocal section goof off or play around during songs just because they know their part. They are expected to sing it as taught, with excellence, every time. Why should it be any different with the band?
HOWEVER, during a band rehearsal, thats a different story. Once the song is learned there, then by all means, let loose a little, try different things, "play" around if you need to. But I don't feel rehearsal is the right place for it.
tj