Master Chief isn't the best at planning ahead. Think about it: When have you not known the guy to land on a planet with just two guns and no secret pouch of ammo? The plan always seems to be some variation of "I'll find some grenades and a Wraith and I'll just figure something out."
If you'd like to buck his example and be fully ready for Halo: Combat Evolved Anniversary with a pre-order, you'll be rewarded with the exclusive "Grunt Funeral" skull. Even if you're not huge on pre-orders, the idea of activating Grunt Funeral and seeing grunts exploding with the force of plasma grenades every time they're killed may be too much to resist.
Perhaps more enticingly, pre-order customers will also be given Master Chief Avatar Armor, so even if they lack John 117's spontaneity they can still share his fashion sense.