<snip>and if it ever get's implemented, i know it's been talked about, but if a fortress gets a reputation for being evil then, the surroundings should be more willing to help then to harm. imagine an invasion being slaughtered by zombie/skeletal trees while urist mcdwarf stops by to give the sacrificial cow and collect the invaders armor. or a caravan reports items missing after traveling over a particularly unruly patch of vines.<snip>
I think this is a bad idea, personally. The whole point of the idea is to make swamps a much more challenging, difficult area to build a fortress in. Having the surrounding environment assist you would effectively negate that challenge. Besides, not all the swamps of death have to or even should be evil-as I pointed out before, there are plenty of creatures available to use, directly or as models, that wouldn't be evil-they're just very dangerous predators.
However, if a race has the evil token/related sphere, then the landscape might help them if it falls into the proper sphere-but it's not so much helping as defending its own, and if the risk is too high then you shouldn't expect any help. Alternately, it might just more or less ignore you and just attack anything with an opposing sphere.