IE is still used on various websites that both Opera and Firefox still have problems with. Namely certain ones that have high security.
I have a few websites that Firefox won't read properly, so i use IE from time to time.
But yes, i do get what you're saying, but most people go with what they know and are reluctant to try new things, because they have no idea if it will make a major impact on there machine, or have any idea on how to install/configure some of these pieces of software.
And before anyone says, it's easy. No, it's not. I have several friends who have no idea how to install most things and rely on me to help out with techie stuff. Same as me not knowing certain things, that they are useful for.
I have been a techie of some kind since I blew up my Atari VCS at 7 or pulled a video apart at the age of 4.