Depending on how hot you cook it, tightly wrapping in foil could result in a great deal of extra cooking from the residual heat of the brisket.
I also don't like to put meat straight out of the cooker into cold storage - that could result in the internal portions being in the "danger zone" for too long.
If you get it all the way to "buttah" tenderness then tightly wrap you could end up with over cooked and mushy brisket. If I get brisket that's cooked to how I like it I will tent it under foil for a half hour or so, then slice it.
I find it easier to reheat slices than reheating a whole flat, then slicing.
Steve - Currently cooking on:
Weber 22.5" Kettle (2X)