or perhaps it could be Gothmog's level 10 power (as he says "Bring out the wolf's head!") that only lasts for a short period of time, with a LONG recharge. it would be great if you are at the enemy's base but not if u plan on crossing the whole map.
i agree with you, that'd be a great idea but is it possible to maybe make a radius near an enemy's fortress that it can't be summoned within so u can't just drop it on top of your enemies base and kick the crap out of him with an almost indestructable unit like njm1983 said?