IE isn't really on the same level as firefox, opera, chrome, etc.
Other than sheer convenience of not having to download a 15 mb file, there isn't much going for IE. I guess there are those FEW things that require IE, but they are mostly microsoft forced things that could run on Firefox had it not been for their enforcement.
It hasn't progressed at the rate of the other browsers either, it seems. The coding is just slower.