Sometimes we may feel that something doesn't really affect us. But does IE's dominance on the Web affect us even though it might not feel that way?
The answer is: Yes, definitely. But the problems with a monoculture on the Web extends beyond browsers! A single point of failure is a bad thing no matter what.
The recent ActiveX security flaws in IE once again show us that a browser monoculture is a bad thing because those looking to infect people's computers will have a single target with a very nice return of investment. And those millions of compromised computers can be used for things like sending spam to the rest of us.
But it goes much further than just IE. One could argue that just about any kind of dominance of the Web is a bad thing.