I've never foiled pork butts, and people eat it like crazy, although to my picky taste buds, it might end up a little drier than if I did foil as evidenced by my experiences foiling ribs. I started out not foiling ribs, and they ended up with way too much chew for me and what seems like possibly a regional preference (NW) that prefers fall off the bone ribs. I would try foiling pork butts, but that would mean getting up at O'dark thirty, and I'd rather just sleep through it at this point (lazy).
I've even been contemplating the need for water, although I do notice with jerky that the extra moisture allows the spices I use (mainly course ground pepper) to almost congeal to the meat, which may contribute to nice bark on a pork shoulder. Sometimes I'll wake up late and the water has run out and it smells like burning fat. I can't tell that it hurts the flavor of the meat, but it smells a bit bitter for my tastes, so I try to keep that water pan filled. Not sure if it really matters though.
I'll tell you one thing though, disposing of nasty pork butt smoker water is a pita.
Your mileage may vary.