Term originated by scrubs who would get consistently beaten to a pulp in Halo: Combat Evolved by superior marksmen. The Pistol was extremely powerful, so only an idiot would choose the cool-looking covenant weapons. Despite their inherent coolness, (what with being alien weapons and all) they were simply inferior to the M6D Pistol. Since they consistently died from the three-shot kill of a talented pistoler, scrubs began justifying their embarrassing losses by calling their (superior) opponents "pistol whores." See example 1.
In halo 2, anyone who is proficient with any weapon and regularly chooses it will be called a whore of that weapon if it is what they consistently use well to defeat players of lesser skill. A player that uses the battle rifle frequently and skillfully to achieve a high win-loss ratio will invariably be called a battle rifle whore. This applies to any weapon in the game, no matter how available it is to all players, or how little/much skill it takes to either wield it or defeat someone wielding it. See example 2.
Me: Crack! Crack! Crack!
God: Game over!
Scrub: O my god you pistol whore! Use a weapon that takes some skill u n00b!!!
Me (with battle rifle): Brrt! Brrt! Brrt! Brrt!
God: Game over!
Scrub: O my god u battle rifle whore! Use a weapon that takes some skill u stupid n00b!!!
Me (with sword): WhoosH!!!! Whoosh!!! WhooSHHSH!!!
God: TRIPLE KILL!
Scrubs (x3): OMG U FUCKING SWORD WHORE!!! GET SOME SKILL U NOOB!!!
A word to describe someone who snipes with the overpowered pistol in Halo 1. And they snipe a lot. That problem doesn't come up in halo 2. (but now you've got duelweilding plasma pistol/smg whores)
*bang! bang! bang!*
"Dammit Brendan! You pistol whore!"