The problem I have is not so much with the defender bonus but with the battle formula itself. Aside from what seems to be constant unannounced changes to it, it is such that the presence certain unit rainbows makes or breaks a fight. When I send my archers with a rainbow, the pikemen get in front and the enemy archers stop further back, which results in them getting an additional free round against my archers. When I don't send a rainbow, that may or may not happen, apparently depending on whether or not the defender has any cavalry and/or cataphract. Compare this

battle32.evony.com/default.html?logfile/20090913/5d/ab/5dab9f24c6df9ee8700bb6a27df790ef.xml where the defending archers clearly get no free shots off on mine

to these two, both occurring three seconds later

battle32.evony.com/default.html?logfile/20090913/80/12/8012b7db132e7443f100f0f49f1e58bf.xml

battle32.evony.com/default.html?logfile/20090913/31/cb/31cb325659025b34384a4a540001d9f8.xml

where mine get destroyed and inflict minor damage only, probably killing some melee units, or else a very small amount of archers (less than 10k), based on the experience. Second/third one was also 90k archers each from me. So apparently having cavalry and cataphract in your city is a bad thing and will cause you to lose alot more archers against an archer attack. This is the kind of thing that needs to be addressed, or at least a clear explanation of the battle formula given along with notice of updates, so we're not left with these nonsensical results.