It won't really sell large numbers of Office, and the fact that less than 1% of people use more than 7gb (the free tier) means that it will make very little money overall.
Office will certainly sell subscriptions of SkyDrive, but they'll be free subscriptions mostly and the people won't really care about it. It doesn't make the core product offering more compelling, which seems to be their core strategy based on the yearly price.
Dropbox has done so well because they aren't just the first mover - they are the standard by which others are judged. With this in mind, Microsoft is making a mistake trying to make an offering to compete with it. They should have just made a deal with Dropbox in the first place.
Dropbox integration would be a fantastic selling point for Office; SkyDrive integration is not.