I'll preface this with the fact that i'm new to my smoker but the first 4-5 shoulders i did i didn't foil. My wife loves "burnt" things so i thought she would really enjoy a good solid bark and she did. But honestly, while good, it didn't seem that my shoulders were as "great" as everyone i saw on here. Not to mention that un-foiled, these shoulders were taking ~15-20 hrs to come up to 203 on my 30" MES @ 225 or so. So while the bark was delicious, they weren't as moist or flavorful as i would have thought.
However, on my last shoulder a few weeks ago, i decided to wrap it with a splash of apple juice. Not only did it decrease my cooking time drastically, ~9-10hrs, but the shoulder turned out so much better than my non-foiled ones. It had good bark was moist and had great flavor. So suffice it to say, my next butt this weekend i'm going to wrap and see how it goes. If its as good as the last one i'm sold and i'll post an update.