I'll give you that there is an additional cost and I understand that as a small business every cent counts, but the cost per year is minimal.
That said, I don't think the separation/unity argument holds much water. You can alway maintain the mobile portal at other urls and have your mobi url point to that. MSN.mobi points to mobile.msn.com and it just makes it easier for the user to remember and type in. PPCT.mobi is a LOT easier to type than
www.pocketpcthoughts.com, even if it does auto-detect and format accordingly.
There should be great concern over domain-jacking, though. That could be a BIG problem if it isn't policed right. Guess we'll just have to wait & see...