Philosophically Unitarian Vs. Institutionally Unitarian
I don't know how typical I am of the elusive identify-as-UU-but-don't-join-a-congregation demographic, but for what it's worth, here's my experience:
I took to identifying as UU many years ago for philosophical reasons, long before I ever set foot in a UU church. My theological views had liberalized out of the orbit of Lutheranism, but I didn't want to reject religion. So UU was (due to its breadth) a useful label for what I believed even though it didn't describe what I (being still a member of a Protestant church) did. I gather from the blogs that institutionally-affiliated UUs are highly self-conscious about their inclusiveness/welcomingness/etc, which is great, but don't let it obscure the fact that there are some of us who formed out here in Enrique's metaphorical Kupier belt*. I've encountered a number of people over the years who don't have much of a religious identity on a day-to-day basis, but have settled on "UU" as an answer when someone asks them about their religion because it's a pro-religion answer without committing them to the doctrine of any particular church or sounding too new-agey (as "spiritual but not religious" would).
That said, I would like to join a congregation, but I'm not about to drive an hour to the nearest one (in Chandler, AZ) every Sunday. I imagine a significant part of the issue is that UU churches are pretty spotty over much of the country, but potential converts like myself pop up everywhere.
*Commenter Enrique had analogized the question about UUs to astronomers' debates over whether objects in the Kupier belt had formed out there or had formed closer to the sun but got flung out due to the gravity of the other planets.