The truth is that it probably goes on a lot more than a sensible person thinks it does. I can recall one WG from a few years back that insisted on protected sex and stated no BB etc., because her concern was getting pregnant as she didn't have a "second line of defence". Found out by accident that she was regularly BB'ing a guy that had had "the snip" so as far as she was concerned he was safe - she hadn't even considered the disease aspect or the fact that he probably used the same tactic with countless other girls. Those that have a second line of defence - the pill, coil etc., are probably more susceptible to getting drawn in to BB as they have no fear of getting pregnant - maybe only with some regulars but it almost certainly goes on.
I think the attitude among many girls is " it won't happen to me" as he looks and smells really clean etc., Age probably has a lot to do with it as well. The younger end of the range 18-25yrs pay higher car insurance fee's as this age group takes more risks generally. A study of the brain revealed that the last bit of the brain to fully mature (at 25yrs) is the part that deals with potential consequences from actions, so that may be partly to blame.
The fact is that BB is definitely out there and only the girl can tell you the reasons why she does it - more money, to keep a regular from going elsewhere, sheer stupidity - who knows. We can only protect ourselves as best we can.