It can get randomly crazy sometimes, and I mostly agree with your idea (though I'd say more like it'd be nearly 100% after say, seeing it about 10 times; 3-4 is a bit too fast). However, it sounds to me like that'd be hard to program into the game. :/

What you'll be experiencing as long as you play BLU is that mobs will often refuse to use the spell you want to learn, even if it's something you always saw them use often before. And then once you learn it, they'll start using it all the time again.