Most of these credits are rolled into your licensing agreement, even with a $20k/year SME open value I recall us getting like 4 tickets free. They aren't just tickets, at 4am in the morning with a botched Exchange upgrade (it was our fault in the end we overlooked something with certs) you have a case manager on the phone (calling you back and a direct number to him/her) then they forward you as required, you end up essentially talking to one of Exchange developers if required. The escalation path, speed, and knowledge are seriously awesome.