By way of background, I've been running pick-up games and convention scenarios for 30-plus years. And, with rare exception, you'll have your choice of several pre-gens I'll be providing as we sit down to play. No, we won't be rolling up new characters. No, you can't play a new character you rolled up and brought with you. And, super-no, you can't bring your home character to play.
So why the hard-nosed approach? Certainly, any GM worth his salt should be able to work these newly-created and/or home-spun PCs into a quickie pick-up game, right? Well, over the years, I've found it to be more trouble than it's worth. And now that we're entering convention season (for me anyway), I thought I'd explain my position on this matter.
1. Time is limited. During a typical convention game, we have 4 hours tops to get from point A to point B. When you have 6 folks rolling up a new character at the table, it takes 20-30 minutes away from our limited game time. (Perhaps longer if someone is new to the system.) So pre-gens cut the PC generation time to mere seconds. ("Here, pick one.")
2. Necessary "detachment." When you take the time to create a new character, you become "invested" in that character. And folks who are attached to their characters take fewer risks. They're cautious. They think things through. In other words, the action slows to a crawl. However, with a pre-gen, you have a "disposable character." A player will do crazy, off-the-cuff stuff. Risks are taken, action is non-stop, and the story moves along. And if they die horribly 30 minutes into the game? Eh, who cares? Have another pre-gen.
3. Additional game-related info. The pre-gens I give you at the table aren't just a stack of stats. I give each pre-gen some background information that only he or she knows about the scenario the players are entering. Through their pre-generated PC, each player is given some vital clue, some needed piece of equipment, or a skill that's important to the adventure. My pre-gens are actually part of the game. Characters brought in from "outside" won't have that clue, skill, or object necessary to further the game.
4. Game-breakers. I've personally encountered this when someone brings their home character into a convention game. Their PC will have some skill that I didn't account for when designing the adventure which totally sends the game off the rails.
"OK, when the game begins, you are captured by the orcs. After tossing you into a cage, they grunt amongst themselves, formulating their secret plans."This also encompasses the +8 Holy Reaver your character possesses, the ninja skills you gained when training at The Ninja Academy, and anything else that pushes your character into Superhuman Levels of Kickassery. I just can't account for every potentiality so it's better if you just leave Cap'n Asskicker at home. The pre-gens all start on equal footing. No one is more powerful or more useful to the game than anyone else.
"Oh, did I mention my character is fluent in orcish? I eavesdrop on them so we know exactly what they're up to."
5. "I'm the star." Folks who bring in their personal PCs have a history with that character -- a history that they'll bring to the table. It sometimes becomes an impromptu "let me tell you about my character" situation. Even worse, it becomes a "let me SHOW you about my character" situation. It's no longer about the adventure, but rather it becomes the "Ragnarok the Barbarian Solo Quest Adventure Hour -- Plus Five Other Guys." They see every encounter as another opportunity to show everyone how awesome their character is, and they'll treat the other players like NPCs. With pre-gens, there are no stars.
Lest you think I'm a bit of an ogre, I will admit that I've made "special arrangements" in the past if someone contacted me beforehand and asked if they could bring a home PC into my convention game. If I can make some prior adjustments here or there in my game script or in your PC's skillset, then it sometimes isn't a problem. In fact, I get a bit of a thrill knowing that my game just became a part of your PC's continuity. However, if you blindly show up with a character sheet in-hand and demand that I let you play it, don't start pouting when I ask you to put him away and take a pre-gen. And if you happen to storm off due to this affront, I won't feel badly for you. In fact, I'll feel I've just done the other players at my table a great service.
/end soapbox rant