Normally, I would agree that it is a good plan to sap the guys MP before you betray him. HOWEVER, the fact that each person can see their MP stats at any given time makes this plan shit. Further more that MC fell for it, is simply a plot convenience which is very forced. I get the story isn't too serious, but just ignoring details that have been around from day 1, just to push a plot, is kind of lazy writing.