It is widely used in maps like Colossus and Beaver Creek. Do not judge the combo by its widely-used name, however. It takes skill to some extent to aim the Battle Rifle in the right spot.
Player 2: Noob combo, biyatch.
The noob combo consists of a plasma pistol and a battle rifle. The user would charge the pistol up and fire it on the opponent. The green blast follows and homes in on the target making escape unlikely. This causes the victims shield to be stripped and then the noob combo users quickly switches to the battle rifle and shoots the victim in the head, causing a instant kill (even if the user misses the head, it only takes a 2nd shot anywhere on the body to finish the victim off).
the Noob combo is used by just as you would expect, noobs. Typically the people using it suck with the legit weapons and want a unfair advantage against their opponents. It eventually got to the point where players would immediately set off to find the 2 needed weapons as soon as a match would start... Thus ruining the Halo 2 experience for many legit gamers.
Noob combo was also used to spawn kill quite often. Despite its overwhelming frustration, the noob combo is possible to beat. The best method is to quickly duck behind a wall or object and once the blast misses, quickly kill the noob while he's charging back up. Its not uncommon to see someone resort to using a noob combo to beat a noob combo user. Fight fire with fire. Or in this case, noob with noob.
In Halo 3, the noob combo was fixed with the simple change of making the plasma blast not home in on opponents. Now the noobs are forced to use real tactics and play fair.
Legit Player: you used noob combo you dumb nub
me: "dude, u had the combo too, dipshit"
Gamer2(red team), "ha, you just got pwned"
Gamer3(blue team), "that's weak man, you are using a noob combo you noob/noob whore!"
player: what? i'm rank 38 and I am supposedly a noob? racist
veteran: yah but i die means u noob and u live n yer moms basement
He just kept usin an old noob combo.