I don't agree at all... all you are doing by increasing the defending force is spreading out the losses to more players. The losses are still the same no matter how well you defend. The defender's strength does not affect the PA at all. He takes out 15K of units no matter what you do.
Lets say there are 10 friends with 1 million defense each. Each sends 100K to the 9 other cities to defend so each city is defended by 1 million made up of 100K each from 10 players.
Now 10 bad guys send 10 PAs to each of the 10 cities... each city suffers 150K damage and each defending player loses 15K in each of the 10 cities. Exactly what would have happened if each player kept his 1 million defense at home...
Your reasoning escapes me. PAs are bad because they can't be defeated, countered or foiled and they reward the user with extra PVP points to boot.