Last winter in Florida on a group ride, a guy showed up on a hybrid with platform pedals and street clothes. Everyone else was on race-ready bikes and prepared to ride fast. I think the reason he was dropped almost immediately was due to his bike. No one said anything disparaging, but it's hard to ride a hybrid with platform pedals at 25-28 mph. Point is, clipless pedals were not required, but were part of the equipment every rider (but one) who showed up was using. It's not a matter of conformity, but practicality.

I use clipless pedals on my commuter bike because I like having my feet attached to the pedals. It's safer, more comfortable and more efficient.