Freedom.

Microsoft Bans Open Source From the Windows Market

Plus, there is already Kontact Touch, or Marble To Go (mobile ports of important KDE applications). There will be for sure a decent amount of open source applications. The Harmattan UI or core apps might not be open source, but very important parts of the stack are.

There is also the standards support. The web and the office market is a rough place due to the use of MS proprietary formats. I can tolerate proprietary software that is compatible with good standards that make interoperability possible.