I just noticed that ubuntu one is available for Windows if you subscribe.
Why is it limited?
It would be great advertising, and help the migration.
The Windows client referred to on the plans page is for syncing contacts. We charge for this because the client is part of mobile sync and mobile sync costs some money to provide due to licensing fees Canonical pays for some of the server technology.
We are currently working on a Windows client for Ubuntu One that will allow you to sync files. It's not quite ready for public testing, but once it is, we'll announce it on our blog: http://voices.canonical.com/ubuntuone