I've only ever done two Sportives. The Quattro and the Gorilla Ride. I do a couple of Charity rides a year, usually the BHF London - Brighton (now £35!) and in the past, the Lymphoma and Leukaemia London rides to do my bit for the charities.
Still, it's an enjoyable day or two out with the chaps that includes much beer in the evening.
There's nothing to stop anyone 'ghost' riding though as they are ridden on open public roads.