I just read an interesting post by Rob Wilborn about the lost art of the group road ride. I am going to copy and paste most of it below. While I don't agree with all of it, most of it is worth reading.
Third, different rides are advertised by average speed, but speed is only one part of the equation. This approach makes speed the sole metric for judging a cyclist, and creates the false impression that a fit rider is a good one. Almost anyone can be somewhat fast on a bike, but few learn to be elegant, graceful cyclists.