We aren't collecting grenades to use at only the right moment in time here..we are talking about making the band sound better. Consider that most bands only use 2-4 18"s for their whole band in a small to medium bar. You don't want those subs up on the stage as a keyboard rig, especially if you don't have subs in the FOH. If the place isn't big enough for subs in FOH, then dont bring them on stage either.
Regardless, this isn't cut and dry because most people don't bring in subs to supplement a lacking FOH. If the FOH has subs, then you should be using them. If they don't and you want them as part of your band's show, I wouldn't just let them borrow mine. YOu have no control over them. I either use their soundman with their own house equipment, or bring in a competent soundman that I know well. I don't let their soundguy use our FOH, ever, mainly for liability reasons. If they allow our soundguy to plug his board into their sound, thats fine. At least I have some idea of his abilities.
Wes