HUDs are good for when it can be unsafe to look away from what's going on around you. That includes flying in formation, close to the ground or in combat, and cycling.
Not really, cyclists try to keep a steady cadence as they go along. It'd be quite liberating to be able to keep an eye on how you're tracking without having to regularly glance down at a little computer thing.
Pilots in a tight formation are very likely to hit someone else if they glance away. Or even hit the ground. I’ve witnessed a pilot crash while flying inverted close to the ground. Certain kinds of flying require an extremely high level of concentration.