The thing is Dave, there is a massive difference between a sportive, where, within certain limits, a rider has plenty of time to make it around what may well be for them a taxing and difficult ride; and trying to stay with a group of fit and pretty hard core riders doing what is for them a easy ride, but is for others on the very edge of what they can manage.
There is also the matter of distance; you may want to do a longer route, in order to spend more time on the bike and to try yourself out a bit more. I, on the other hand, may only want to ride two thirds the distance you do. We can both enter the same sportive and both be accommodated, as there will be two or three distance choices. With an RB ride the route is the route, nothing wrong with that, but if that route, either in geographical terms or in distance, does not suit me, then I won't attend, as I don't want to be left behind holding everyone else up, and be made to look a pratt in the process by running out of steam when the rest of the ride have plenty left in the tanks.