I think in a lot of cases, Curves is targeting the ladies in the crowd who would never otherwise go into a gym. Its a marketing ploy definitely, but I think the reality is that its better to do something (e.g. Curves 30 min circuit training) than nothing at all. If they are uncomfortable around weight rooms & guys, this is definitely one way to approach it.
For me however, I friggen hate that stuff. I don't like to be around women who whine & bitch about their weight. I enjoy the social atmosphere of a gym and I really enjoy lifting to see what I can accomplish & it just feels so damn good!