I think that's actually pretty smart on nokia's part.
So long as Samsung and HTC Windows Phones offer the same experience as Nokia, they've caught up already. In fact, they'd "caught up" before Nokia even shipped! So, who's playing catch-up here anyway?
- custom apps ( even if it's not on the phones by default) - marketing. once you've got your phones so much associated with and OS, it takes some time to convince your existing users that they'll need to change
However, that strategy seems to have changed, with Drive being licensed to all Windows Phone devices, regardless of manufacturer. So they do lose a major differentiating feature, but they gain something else: the ability to make money on any WP device sold. I guess they prefer to differentiate on hardware going forward.