Ok, so if you have a top #1 alliance, and a guy with 2k presteige trys to join you, do you accept because presteige is meaningless?

i think not

If you want to gauge the general strength of a target, or alliance as a whole, do you completly disreguard their presteige because they might have taken 1 or 2 10% losses?

nope

you are wrong.