Take border-radius for instance. It is 'fluff'. It makes no functional difference to the page whether it is there or not.
Now, for Moz and Webkit, I get nice looking rounded corners for negligible effort. In order to duplicate that look on IE there are any number of ways to do it, but all of them involve enormously much more effort, and are much more fragile (anything where you need pixel perfect positioning is fragile and easily broken by different browser versions).
Additionally, the longer WE support IE's blatant disregard of standards, the less incentive they have to adhere to those standards.
