Way too sweet for my taste, Steve, but I do use a little unsalted butter melted in stock for brisket. Spares I use a juice blend but on the very rare occasion I cook backs I use a little unsalted butter melted into juice or stock, depending what I'm doing with them.
Dan-- Smoke flavor comes from smoke but the smoke does not need to be visible (if that makes sense).
As Jim notes, butter will not add moisture (I wouldn't use margarine for anything if you paid me) but it does decrease evaporation of moisture (read: water) and can speed cooking a bit (evaporation cools meat so it cooks a bit more slowly). Fats also contribute to mouthfeel and flavor--but you need to be careful here and think about what your shooting for. E.g., a butter-in-the-background flavor with a smoother slightly fatty feel in the mouth when eating turkey breast might be desireable (it is to me which is why I mix rubs with unsalted butter or a butter/evoo combo for under the skin), but it might not be what you're looking for with brisket (it might, I'm just pointing out considerations).
There are alternatives, however. Tallow and lard are possibilities too. Personally, I only like the meat's moisture as a rub adherent (and use salt separately to draw out moisture as noted in any of my riub recipes) but try different things and see what works for you. Note also that different fats can be combined. If butter is too much alone, e.g., try mixing 1 part butter with 4-5 parts lard or tallow. Another point: butter and most margarines melt at fairly low temps. Lard, tallow and shortening are stable over a wider temp range. That might be important to you in sme cases (e.g., for use on an overly trimmed brisket flat).