I think Microsoft's interest is partly that they need to offer a centrally-manageable browser to support their corporate customer base. How dumb would it be to make the Information Technology staff go and
manually deploy, audit, enforce & maintain browsers on every computer in the fleet? Well, that's how every other Windows-compatible browser works, except for Microsoft's own. By making a centrally-manageable browser (IE5 through IE8), they give enterprise customers an additional reason to stick with Microsoft-based OSes and domains.
Also, by guaranteeing a support lifecycle, they ease the customer's mind about "well, how long is this going to be supported for? Oh wait, Microsoft spells that out right here, it's supported for the life of the OS it came out with. Ten years, unlike these come-&-go open-source ones that keep yanking the rug out from under us after 18 months."
That's my take on it, anyway. Having had to manually maintain a fleet equipped with a certain open-source browser for a couple years, it certainly stuck in my mind
