Some people will buy WP just to have a Nokia phone. These numbers will make (hopefully) WP relevant in the market (10% market share?).
If this goal is reached, then WP, which is not necessarily worse than BB for example, will be considered as a player on its own, gaining traction for what it is (a middle-middle OS with powerfull developpers backing, ie something that could work).

It is very important for both Nokia and WP that WP reaches this critical market share, in the relevant markets. And that is why they are pushing so hard in these markets (even against their own handsets), and why they cannot say that there are other options for Nokia. It doesn't really mean that they don't, though.