Another way to look at it, in terms of tactics, it’s often better to attack after your opponent has used most or all their MP. Also, a second opponent might not be in range, or might be at long range, at the beginning of the round, but might move closer in the middle or by the end. If you have a short weapon, it’s better to attack at melee range than onset range (you’ll get a higher bonus).
Depending on the Martial Feats you have, MP could also be used to move away so as to avoid a grapple (Sidestep etc.). Which is pretty important if you are outnumbered.