That's like saying that you need to make your website work with Opera, when most people don't bother. In this case, Microsoft is in Opera's place. I'd say it's pretty safe to ignore them.
Kinda a bad analogy as Opera is pretty much on par with Webkit/Gekko. Sure it basically shares the same JS-based quirks as IE, but it has a much more robust and usable error console so I welcome it.
Actually, that means the answer is even clearer for Windows Phone - since the differences are greater, it should have a proportionally larger share of the market to make supporting it worthwhile.
