Many people do not care if something is open source or not, rather wether it is free.
However, many people do.
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For me personally, there are 4 reasons not to use IE:
1) Duh, it's a Microsoft product! Using IE is a tacit approval of the Microsoft company and the Microsoft way of doing business. Of course nobody knows I am not using IE but me, but dammit, it's important to stick to your principles at all times, even when nobody else is watching.
2) Standards bludgeoning. Of course not every browser can be 100% W3C standards compliant, because the W3C changes their minds about things every day. But no other web browser is making up their own bits and pieces of HTML for their own specific use. IE has all these little bits of code that it made up which go against standards. And some of their handling of XHTML is mind-boggling. I can provide more specific examples if you don't know what I am talking about.
3) Web annoyances. In reality, these aren't IE's fault - it's the stupid damn web designers preying on the fact that IE is so popular. Mostly these are pr0n sites who try to force downloads or redirect or take control of the browser. Probably Linux and OSX have shielded me from as many of these as not using IE.
4) I just don't fucking like it. And nobody can tell me differently. This is one of those points that means nothing and everything all at once. The greatest program in the world is useless to me if I can't stand to use it. So there's why I like Firefox. And there's why I am using SeaMonkey right now. Opera might be good, but something about it just rubbed me the wrong way. And IE makes me wanna slit my wrists everytime I see that Favorites menu up there where the bookmarks are supposed to be. Well, perhaps growing up on Netscape Navigator has something to do with it.