We've had this discussion in my band, already a 5 piece, do we add a keys player or use samples? What I enjoy about this band is the spontaneity (thank you spell check, autocorrect thingy) and the danger that if we are not on our game it can all fall apart. I don't know how we would react to a punter doing something funny or the drummer falling off his stool, etc if we are playing along to something.
I've got an 80s band starting up, first go with a proper singer next week. Me and the guitarist are regularly in touch talking about how we cover certain arrangements where keys are very present. This has been a good challenge, but it's time consuming!