1. Terran does not need another Marine-wannabe.
2. He would still be worse than Marine. (for is there any force in the world that could ever match mighty marine?)
3. Ghost is a spellcaster and it should stay like that (even though its in quite pitiful state now)
4. He's too expensive to become simple front line soldier and it does not fit the unit completely.
5. What about Nuke?