I always use mustards with butts, and often with ribs. I'll probably try it with chicken next time around.
It imparts no taste, but works wonders for making the rub adhere to the meat.
Since you can't taste it, I use cheap yellow mustard and save my Dijon for other things.
I just squirt it right onto the meat, then spread with with the palm of my hand. I then load-up a cheese shaker (like the kind you see in pizzerias) with my rub, shake it on, and give it a gentle pat down. Makes for one beautiful butt.
As an aside, my favorite rub is "Magic Dust." Recipe can be found Mike Mills' book, "Peace, Love & Barbeque."