Oh, so I misunderstood your point. Yes, you are right, but I think you can play on the conditions (target, trigger) to say 'the spell (which automatically works, because the spirit is bound) commands the spirit to do <programmed> task, in that case, 'cast fireblade'. I agree this is not written as such, but Scotty's comment gave me the idea.
Yes, of course, but the point was to have an automatic casting of an active (in that case, Fireblade) spell. I think I will stay with Bladesharp.