Funny, I was going to post a question about this same thing. My Tuesday Worlds is a really, really tough ride, but the standard advice (which I've always blindly followed) seems to be that you need structure. If a group ride stresses the right zones and especially since it's race specific, why is it not perfect training (as a substitute for intervals)?
Because it's usually too easy. Too much Vo2 max mixed with freewheeling. Your critical power doesn't improve. The kind of efforts needed to improve often involve going to the limit which would mean potentially getting dropped on a really fast group ride, or going off the front on a slower group ride.
Think about it, if you have a group ride for 30 guys, how in the world is that going to be a good training program for every person? It isn't.