That was my argument vs a member of my faction, they seem to think that because respect diminishes every attack that exp would too - however there is no way to test this that I know of
Confirmation: After literally 16,000+ attacks,with many multiple hits on players, with wars going for as long as a year (eg NINJA AOW Victory), hitting the same target over and over, in attacks or defends (8,000+) does indeed give less experience and respect per attack incrementally in both cases period, takes a bit of perseverence.
it has been known for years that paying for sluts in large number vastly reduces level xp gained for each defend after not so many so its probably worth paying a level 100 to attack and lose just a few times a day.
i dont recall seeing any posts from admins saying the same applies to level xp gained from attacking though.