-10% evasion mean reducing by 10% the accuracy need to hit the target
It better than +10% of accuracy on the group

Since the accuracy need is always higher, the reduction is stronger.

...

And no having a bard using that skill mean that he lower his DPS...
The group should have the accuracy requirement

@Slappah not that much... That rain is a blessing on Alex2 XD and many other fight where healers dps