"It's main purpose is to stop enemy units not to kill them completely"
That may well be the way you find most useful for the
trap spell but that has nothing to do with the idea of achieving the
trap medal. The medal and it's bonuses are a reward for using the spell
creatively! The idea that you're proposing requires little creativity and is unfairly biased towards mages.
If that's the way you want to use it then fine don't worry about the medal. If you're prepared to use it creatively then the medal rewards you for that. Pure and simple.
IMHO, the Grand Strategy and Trap Medal's feel great every time you level them up because you had to work hard to get there. On the other hand, the others are nice when they level but there's just not that same satisfaction.
BTW The Trap Spell for a mage with high intellect can deliver well over 8000 damage and poison! That certainly is more than just a friendly grab around the ankle!
EDIT: Damm DGD got me again!