I don't read much urban fantasy but I am rather fond of Jim Butcher's Dresden Files, an adult fantasy/mystery series about a wizard gumshoe who is periodically called on to save Chicago and sometimes the world from the forces of evil. Not long ago I checked out his website and was reading through his older blogs when I came across one where he'd posted some of the critical reviews he'd gotten over the years. One of the reviewers said something that really caught my attention. He said "why not actually make the demons good and the angels evil" as a way of adding interest.
There is a trend in literature lately to do just that. I think there are several reasons: 1) writers want a plot twist that will surprise their readers, and 2) it's considered avante-garde.
The problem here is that good and evil aren't just words, they're actions. If you write a story about an evil angel, it isn't an angel anymore. The creature is, in fact, a demon in disguise. It's the same as if you write a story about a policeman who works for the mob. He may have the uniform, but he's not representing the law.
I like stories where the author messes with appearances; kind goblins, or evil princesses. I think they're an important reminder not to judge things by their surface. But I think it's important not to say that good is evil, and evil is good.