Tough briskets, dead horses....I'll weigh in.
My best briskets haven't been wrapped in foil until they came off the cooker, and even that'll mess up what little bark it has after smoking on my wsm with water in the pan. I use to think that foil would keep 'em moist, but I think it speeds up the cook more than anything...and I think if it's a decent piece of meat to start with, it's hard to beat cooking 250-275, and a good long rest in the foil.
Anyhow, one of my best ones yet (13 lb. choice from Sam's) cooked all friday night on the wsm over a couple big pork butts with water in the pan. It and the pork butts were all done at the 12 hr. mark, earlier than expected so they rested all day in a hot cooler. Well, the bark was pretty mushy so I put it in the oven for a bit before slicing and everybody said it was really good. To ME though, it was a LOT better for supper last night, as usual.
I think as long as you don't overcook and give it a good rest (Dr. BBQ recommends 2 hr. at least) you don't need any foil while it's cooking. However, foiling sure makes the brisket cook more predictably, especially if cooking at higher temps.