Only Microsoft has the sources for their stuff, so the only way we can find and report problems is when they happen or if we poke and prod it to test it. With open source software we can find where the bugs are in the source and get them fixed.
Because I am a corporate whore and to achieve my delusions of CEO grandeur, I succumb to the nuances, subtleties and nooks and crannies of empty suits everywhere.
No, really, I find that the formatting isn't always identical when you move work back and forth between Word and OpenOffice. The rule of thumb I usually live by is: Microsoft for the boring stuff, Gentoo for what's real.
I know you can use WINE, but I still end up using XP. When I do have to show someone something on my machine, XP has the merit of user familiarity.