To me this is kind of a yes and no answer. Yes, they are a waste if you dislike using them to destroy the enemy's city/fort buildings, generals, merchants, diplomats, and assassins. If you dislike that kind of playstyle then you'll find assassins a waste of time, money and effort.

If however you like that kind of playstyle you'll find them quite valuable once you got the assassin trained up a couple of levels or more.

Basically you'll either like them or you wont. Simple as that.