Do you use agents yourself? They are pretty powerful. I can't even make some sort of invasion without a spy and a hero around. Their abilities to kill commanders before battles or prevent entire armies from taking part in a battle are pretty great and useful.
I've used agents since STW 1 (never got tired of the ninja cut-scenes), but they were always a very minor part of the game, for me. To need agents in order to carry out an invasion other than opening the gates prior to a siege is absurd, IMHO. And disabling an entire army is even more absurd...

There are some things that plague the game now and by now I am pretty certain that they cannot be fixed without doing a few things all over again, which CA already said they wont do.
And this is, despite the big improvements made by CA, the crux of the matter for me. I simply can't see myself playing the game several months later, after the novelty of it wears off. For 15 or 20 dollars US, I suppose that might still be worth it. One can blow much more than that at a movie theatre for a single evenings entertainment