A handful of possible reasons, I guess. OSM data has the potential to be much richer than anything offered by Tele Atlas/Navteq - footpaths, cycle stuff, POIs. Like Andy says, it's often present in parts of the world which the existing suppliers don't cover. It's obviously much cheaper, which is good for the big guys in itself, not least because it means they can force down prices from their existing data suppliers. And strategically it doesn't make sense to be over a barrel to only two possible suppliers, especially when one of them (Nokia owns Navteq) is a rival in some areas.
But I suspect a lot of it is simply because OSM is "hot" right now and, you never know, this might be a way to get one over on Google.