It's an interesting mechanic; you can use it to allow strategic use of your mp in game. Of course, the trade off would be less mp for other things, kind of like turning someone into a berserker for a couple turns by sapping their mp.
If MP is also the resource used for casting skills in the first place, it becomes a very weird mechanic because you have one resource trying to gate two entirely separate things at once (extra effects and skill use). I've never seen this implemented effectively, except for games like Race for the Galaxy where you are making an either/or decision with your cards (which are the resource).
However, this could be a good mechanic for warrior-type characters that don't use MP as the resource for skill use.
Despite time and all the other things I am working on, I decided that I am going to start a Fire Emblem-like project. Follow your dreams! I'll just have to balance hours efficiently. I'm also not making this as complex as my Might and Magic inspired project, so it will be a much easier project to work on.
So I've taken to doing a test of the game, and making a list of everything I think of during the test to add/fix. Over the course of the night I check them off as I do them, and by the end of the night I always feel like I've accomplished something.