I think you may be discounting businesses that actually purchase a lot of Office licences. SkyDrive can be a way of providing file-syncing and sharing that's 'good-enough' for their employees/company. (edit: by which I mean it's a valid strategy to try and protect your turf).

I can see an analogy here with Sharepoint. My (limited) experiences with Sharepoint have been annoying and tedious yet the fact that it integrates with existing MS products and is 'good-enough' has made it very successful for Microsoft [1]. I'm not trying to suggest that SkyDrive could be a $1b business but that we shouldn't write it off so quickly.

[1] http://www.businessinsider.com/microsofts-billion-dollar-bus...