Agreed with several posts above...
Foiling reduces the formation of bark, keeps the bark a bit softer, and cuts down on overall cooking time by about 1/3 or more (time is dependent on foiling temp) by steaming in the foil. The lower the foiling temp, the less overall time to cook. If you want to experiment sometime (when you're ready for a long-haul smoke), keep the butt on open smoker grate the whole time...the bark (if you're into the heavy bark) is fantastic if a rub which can handle the long-term heat is used (low/zero added sugars). I've smoked several without foiling recently, and we're hooked on it now. The last one I pulled from the smoker (approx 9-10lb...broke my scale earlier and couldn't weigh it, and it wasn't labeled...from a case purchase) took 25.5 hours to reach 200* @ 225* smoke chamber temp (that's why I say to do it when you're ready for it).
Eric