I would think that a pork butt has enough fat in it already, and it is a large mass of meat. Ribs, are a tad more lean and are prone to drying out, hence why some people employ Parkay or butter. I personally don't use butter or Parkay (ick) in competition on pork or ribs. I will put brown sugar on my pork and a scant amout of apple juice at the time of foiling and that's it.
Conceptually: if it's for flavor, then yes I like the idea of trying it. However, didisea makes a valid distinction between butts & ribs. Butts have plenty of fat, even with the fat cap cut off.
If you're going to try it, then I'd definitely trim more fat off the Butts first.
Even with ribs, I don't add butter very often. I just find it's not needed. Not to mention, I don't like the heartburn associated with the extra butter.