Fun fact, we have Romney's 2010 and 2011 tax returns. McCain only provided two years of returns too when he was running. It's not a hard-precedent to provide "everything you have", either tax records or birth certificates or transcripts or letters of recommendation... But looking at it from a politician's perspective: if it can only help you, fine, release it. Otherwise why bother? Unless the damage from not releasing exceeds the damage from releasing (which I think is becoming the case for Romney), it's just unnecessary information.