Why run IE9 and Firefox or Chrome?

It’s clearer now IMO that undisputedly Internet Explorer is the only real browser for Enterprises now that Firefox is on such a short release cycle like Google Chrome.  I mentioned the support lifecycle and patching as one of the major reasons in my - why IE9 is the best for Enterprises article.

This video (below) corresponds to the article - TechNet Radio: Top reasons to Deploy IE9 video(download formats at this link).

The question I have for enterprise customers is – why do you have Internet Explorer AND another browser installed in your company?  Is it because you let users install their own browsers/software?  have an outdated version of IE?