As BLM (lvl 55, 733 MPs) I'm annoyed when people try to explain to me how to manage my MPs... when the party has not even started (i.e. they have not witnessed yet how I manage them.)
There is the common misconception that Ancient Magic MB is a waste of MPs... which it is not if you know what you are doing (at least at level 50-55, it might change later on with III and IV elemental spells.)
Let's take an example with Ice based spells (since Distortion is a common MB at that level ranges and crabs a common mob, weak to ice.) Blizard II (77 MPs) MB hits for 340 when unresisted, while Freeze (307 MPs) MB hits for 1260. So to beat the Freeze damage would have to do 4 Blizard II MBs (288 MPs)... of course you won't get 4 SCs in a fight, plus you have rescast time, so you end up tossing different elemental spells and not getting as much damage... so you end up a) spending more MPs, b) taking more time to kill the mob. Of course you have to time your MB / SC so that it's just the amount of damage you need to kill the mob, not less (or you will get hurt) and not much more (or you might be wasting MPs.) But a good BLM will know when.