As I said, I am in full agreement with the vast majority of UU principles, and certainly do think they do a lot of good. I would never exclude anyone from a worship service of any kind, be it a UU service, a Catholic one or an LDS one.
I'm really strugglng with knowing how to say this without offending anyone, but to me, part of any worship service is the feeling one gets when surrounded by people who all share a common understanding of who God is (whether that understanding is accurate or not). I try to imagine sitting next to a self-proclaimed atheist during a very spiritual worship service and listening as a prayer is offered, thinking, "The guy next to me is thinking what a joke this all is." It just wouldn't be the same as if I were sitting next to someone whose feelings were more like mine. My brother-in-law is an atheist and my sister is agnostic, and I would very much love to see them attend a worship service of any kind.
All I'm trying to say is that, to me, religion ought to be about God. If it's not, and it's a social organization made up of people who may very well be as loving, compassionate, and giving as anybody else. But it's still a social organization.