I would have to agree. He may have the stuff to be a top 5 starter down the road, but if the Rangers move him to the rotation this year, there is no way he goes beyond 150 innings. And that is not top 5 starter material. Besides, as a starter he could get exposed. He'd be like a rookie all over again.
I think it's a moot point, though, because I don't see them pulling what the Yankees did with Joba.