Pavel, your idea would make armor upgrades for trade ships obsolete. No matter if it is CAU8 or AU1 Stork, pirate would be able to harass traders to such degree that they would lose motivation to trade. As Lane Hacker, I usually give an optional. to yield with 10% hull remaining. If the trader would lose 90% of cargo, that would mean there is no more reason to survive as respawning would be quicker and cheaper.
I want to see people fighting for their survival too, thus the whole reason behind Ageira's CAU8 Bisons that puts a new challenge for criminals.