Init2winit is correct. The only real reason I ever do the break in is so that I seat the top ring to the PISTON MANUFACTURERS specs. If you read the literature they give you with the pistons, you'll see the amount of time they ask for "proper ring seating", but the only reason I do it, is so that I've done what they've asked...just in case I have a problem later. Buschur's race motors require NO break in, as did AMS's, BJ's (back when he was around), etc. In fact, I'm tuning a car tomorrow at BG that has a Buschur motor in it, and it has NO BREAK IN. Fast idle, change oil, strap on the dyno, and beat on it.
Dan knows all these tricks too. However, he's never in any real hurry to tune a car with a motor he just built, so why not let the customer drive it around for awhile and then come back? Also, not every car that comes in has a motor built by him, so he has to account for other people fuck ups. This is the reason he's very cautious about it.