Can't you just deactivate/activate to reset mp on your automaton? I suppose there are situations where that's not convenient to do, but most of the times it's doable.

Altough I agree it would have been better if Spiritreaver could have made a smarter use of cure spells, didn't want to whine back in the march update because I was so happy about the new stuff, but I always found it particularly stupid that it never uses cures under V unless it has not enough mp for it (in which case, it uses IV or III or whatever else)