The "experience" doesn't have to come from playing a specific instrument though. Obviously you're not going to play everything you write for. The "experience" comes from actually working with instrumentalists and learning how each instrument you use works, studying rep., trial and error (with the instrumentalist trying it out), understanding its possibilities, etc.
Performance experience is nice and all. It helps when coaching rehearsals, I'm sure. But then again, I do fine running rehearsals, so maybe it doesn't matter.
Anyway. Yeah.