John,
If you have read any of my posts you'll know that I'm a big pimp for the pad. (its not really a pad is only 5/1000" thick and its Phase Change Thermal Compound). CHOMERICS T725 is the brand used by most of the AMD approved hsf manufacturers, but AMD has a couple of other brands approved as well. Nevin, who makes Arctic Silver disagrees with AMDs approval but they don't approve his stuff.
While initial temps may be a degree or two lower with good thermal grease, many of them that are Silicone based, dry out under the high temps that TBIRDS produce. Its a fact that PCTC stays stable much longer under many thermal cycles. Also most people just don't know how to apply grease correctly (maybe this is why AMD does not approve Arctic Silver). Nevin has speculated that the pad may also act as a cushion to help prevent chipping the TBIRDS fragile core, could be.