Interesting...I have three reasons for always trimming. (just my theories, folks...not claiming it's Gospel)
I've only done two brisket flats, never a whole packer or anything. However, I've done a TON of pork shoulders, and since they are both huge chunks of meat with a lot of fat on the outside, for the sake of argument - let's say they are similar enough to make direct comparisons.
Now when I do pork shoulders, I ALWAYS trim off as much fat as possible, and whenever I get around to doing a whole brisket - I'll do the same.
In my experience, the fat doesn't add THAT much flavor - but it does provide one advantage - it can insulate the good meat against heat spikes. A properly cooked, very trimmed-up pork shoulder will be just as good as one that's not trimmed...once again, when cooked right!
What the fat layer DOES do - is it prevents the rub from flavoring the meat directly underneat it! When I used to leave the fat cap on, I noticed as soon as the meat was done - the fat just slides right off the meat...which is what you want. Except it takes all the rub with it.
I trim the hell out of my pork shoulders, so that every square inch of surface area has rub sticking to it, that will form a nice bark. You don't get any bark where the fat was left on during smoking.
Reason # 2 - after watching all the episodes of "Pitmasters"...notice how all teams trim their meat up good. If contest winners are doing it, so will I - if I enter a contest, that is.
Reason #3 - Just about finished reading Mike Mills's book, "Peace, Love & BBQ" He trimmed for contests and he has 4 or 5 grand champions. If it works for him - it might work for you!
Great book by the way...