They are nearly always required. It's one reason that I've never ridden one; otherwise it would be quite fun to burn off a few wannabe racers on a fixie or a 50s clubman.
Apart from the helmet issue, I don't know why you are so sniffy about sportives, Roger. I'm also not sure how many riders you would "burn off" on your 50s clubman. I've just looked back at the 2009 results for the White Rose Challenge, one of the sportives that I rode last year. It was a ride of 185km, with 3600metres of climbing. Of the 295 riders who finished the event, 20 did it in under 7 hours, 98 did it in under 8 hours, 206 did it in under 9 hours and 260 did it in under 10 hours. That's total time, of course, including stops. And anyway, so what if you could do it faster than some of the other riders? Isn't the fact that 295 riders enjoyed a challenging day out in the Yorkshire Dales something to be welcomed, not sneered at?