So when IE 8 came out, we left the styling needed for IE7 in the ie.css file, even though it wasn't needed for IE8. That made one less file that an IE browser had to load since IE7 and IE8 styles were mixed together in one file.
However, now that IE 9 is out, leaving the IE7 styling in IE makes it seem like these styles are important when, in fact, they are increasingly becoming irrelevant. There's also nothing in the ie.css file that shows these styles are only needed for IE7 and earlier.
We should rename the ie.css to ie7.css so that themers are aware of when and where these styles are needed and can better decide what to do with these styles for their own websites.