Limitations to training on singlespeed?

To answer mdcc tester's point about the need to have similar abilities, I'd say that once you have mastered the technique of group riding, which is basically being able to follow a wheel, it is quite possible to have a range of strength within a group.

For a steady ride, of course. I could sit on the wheel of much faster riders than myself if they weren't actively trying to drop me, but modern training is all about intervals. If I do 4 minutes towing somebody good, I'm not going to be able to hold their wheel while they do their fast effort and I'm supposed to be resting. If I go out with somebody substantially slower than me, they will be shagged out following my wheel while I do my fast bit and they won't be able to put much in when it's their turn on the front. It works on the track when there is a tempo group above the blue line and you do your interval on the black line before swinging back up into the group, but on the road you end up spread all over the countryside, or alternatively nobody gets the workout they need.

Group rides have their uses, but mainly they are about practising group riding, not about developing fitness beyond a very basic level of aerobic conditioning, improving pedalling efficiency by creating muscle memory and, for people who need it, burning off some of the lard they put on after the end of last season, and even for that, high intensity intervals seem to be a more time efficient way to drop some weight.