Buildings don't move. If it moves, it's not a building. :P
I think the idea is nice. I'm thinking about things like defenses built on uneven terrain; cloaked flying mines releasing deadly things if units pass beneath, or anything like that (which would probably totally screw uncloaking, 'cause of the sphere).
The expensive Krow is not yet unique at all, I am expecting this to change anyway, so there might be a big heavy slow flying beast with special abilities someday. :D