Would allowing BLM to cast while moving break the job?

[Felis]

I didn't think it was too OP back when we could move and cast. I seriously think they only changed it so we couldn't speed run Dzmael Darkhold back in the day.

During the time where it was possible to cast while moving MP recovery only started while standing still. Move too much during a fight and you had MP problems. Oh, and you stopped the moment the spell animation started and could only start to move again after the animation ended.
What I don't understand is why the cast get interrupted when the target move behind you before the cast end. This was not in 1.0