I don't have any science books to back me up, but I'm of the mindset that searing beef seals the juices in, plus it gives you a really tasty bark. I've been doing it for years with my pot roasts in the DO, plus I've seared beef when just grilling. All turned out wunnerful!
When I heard of SmokieOkie's method of doing briskets, I thought that made sense... and it did! I do all my briskets his way now. I seared a beef shoulder roast before smoking it... Damn that was good too!
It's the same result you get when deep frying a turkey... when you lower that turkey in that hot oil, it sears the outer surfaces and seals the juices in.
All I can say is, if you haven't tried it yet, then you must!