As for professional fighting games and such... those are usually played with arcade sticks or, at the very least, gamepads, so you can't ever press both left and right at the same time anyway.
I have to agree that pressing 2 keys at once would cancel each other out; it's perhaps a remedy of translating a joystick into keys. If you were to yank the stick both ways it'd end up in the center.
dungeon fighter proves this is false. in this game (a professional mmo beat em up, televised live in korean tournies) they use the keyboard (you can find vids on youtube). and for obvious reasons that i had previously stated (well atleast i thought it was obvious since ive actually played these types of games b4) when you press two opposite direction keys, the last key is the direction you move in. it has nothing to do with joysticks or w/e, its about proper coding for a fighter. you cant be lazy and allow silly stuff like character stopping momentum when pressing a key by accident. that might be acceptable in other genres but not fighters.
i made a complicated fix for this, but i dont know if its still there. i might still have the caps for users. however the events were setup to account 4 everything i could think of with key presses so it was around 40 events. it was mostly just repeating 10 events for each direction. but for a more recent prototype i just put the directions in a variable and work from there. when its in one variable i can control directions like a state. which made more sense sense direction keypresses in a game are like a state anyway.