Okay... many can't go as low as 5. But whatever their minimum is, I'm guessing it isn't ANYWHERE near 50%.
Deliberately sending units that you know will lose can be called a "fake attack".
But deliberately sending a large force that you hope will win, but you don't want to lose all of it, is hardly a "fake attack". It's a *jab*. You are either going to knock the enemy on its butt, or he's not going to go down easy.... and if so, then you want to step back and re-think the situation.
A person who sends lots of fake attacks is doing so (presumably) according to a plan.
A person who sends, say, THREE "attacks with drawal at 50%" is just burning up his men in slow motion. After the first withdrawal, the attacker is either going to dramatically increase the size of the next sortie (if he can), or he's going to send a minimum force as a fake attack.
I can imagine very few scenarios where a person would send a same size army more than once as a "stop loss" style attack.
In conclusion: this is not a technique that can be abused very easily.
It can only be used...rather than abused. And the city being ATTACKED will be delighted to find out that he was attacked by someone who had a weak stomach. It may just trigger a very strong COUNTER-ATTACK!, if not an uneasy truce.
Warm regards,
Lord Sandman