I miss in Halo 1 when you could hit Melee, Grenade, Melee (BLB) and kill someone instantly.

I miss in Halo 2 how you could melee, reload, melee,(BXB) or melee, reload, and shoot(BXR) with any headshot weapon.

Oh wait, Halo has always had ways to abuse melee.

I think the problem you guys have with melee is that you're all just butthurt about the assassination animations. Keep an eye on that radar, or use armor lock and teamwork to avoid melee deaths. Take the game less seriously and you'll have a LOT more fun with it.

Or go play Counter Strike.

The competitive play comes into place when you have crazy awesome teamwork. Halo puts so much emphasis on how strong the player is, and makes you FEEL strong, so you feel like you don't really NEED a team. But if you're rolling with a proper team who can assist fire and call stuff out properly it changes everything.

Same thing in Halo 3, same thing in Halo 2, same thing in Halo 1. That's where the competitive play is and the skill curve is, is just with teamwork.

That being said, I don't care enough about Reach to dedicate the time to become that great at it, because that's just stressful. I just shoot kids, punch kids, and toss nades at em. Don't get all butthurt when you get beat down because YOU didn't check your radar, or because YOU didn't adapt to the situation. It's YOUR fault that you died, not Bungie's. If you see someone coming up to you to melee you, guess what?... YOU CAN PUNCH THEM TOO.

tl;dr
From an actual design standpoint...

I'd say the melee lunge is there because it's very difficult to tell precise distances in 3D space when you see this 3D space on a 2D plane. Your brain can't use it's normal ways of determining distance using depth and so you have to rely on size of objects in relation to the objects around them which is a lot less accurate. The lunge helps bridge that gap and that's why it's become so successful and common of a gameplay mechanic.