Oh, yeah! I think it is MUCH more common than is let on publicly. There are several parents who made such "confessions" to me. I tend associate co-sleeping with crunchier families... but the fact is that a
huge segment of the population co-sleeps and many of them (if not most of them) aren't that crunchy.
Some co-sleep because they breastfeed, some do it to promote better sleep for everyone, some just enjoy the intimacy of a family bed, and some co-sleep because they only have a limited space and it makes the most sense logistically. There are a million more reasons, just a diverse as the families that practice co-sleeping. Honestly, I think the majority of parents who's kid has serious sleep issues, at least at some point, try co-sleeping. Sometimes it's the only way ANYONE gets any sleep.
Mostly, though, co-sleeping is just feels like a natural way to care for your children during the night, and it doesn't take a granola mom and dad to figure that out.

So, yeah, I think there are a LOT of closeted co-sleepers out there!