I personally think
Evan Rachel Wood is a reason why I would NOT use
Evan on a girl. She just rubs me the wrong way.
I really like most names that are historically male (like
Ashley,
Lindsay, etc) to stay on males. And though there hasn't been much of a crossing over with
Evan, it still is entirely male to me.
I love, love, love
Evelyn though and would much rather see that on a little girl. (Though I'm biased; that's my grandma's name.)
All this being said, it's not the worst idea in the world. The nickname Evvy is cute.