First of all, if Wii games can be modded or play over online with Action Replay-type devices, then you won't be worrying about playing Melee Online, you'll be worrying about people cheating or your Wii freezing up.
Mario Kart DS - There are so many action replay users online. You'll notice when you see 10 Blue Shells start flying at you(and this is before the race even started), or dozen of items being used in a row. Anyway, your Nintendo DS could freeze if someone spammed Blue Shells.
It's up to Nintendo to make sure their online services/games aren't easily "Action Replayable".
Though, it shouldn't be a problem. The DS doesn't have any updates or anything so they can't patch things like AR devices out. The Wii does have updates.
So as far as I know, the "modded Brawl" will only be limited to "local" players and not over online play.
Now, I understand how you feel but a couple of things I'd like to point out:
1. This is the internet, there will ALWAYS be people like that. Whether you're playing online Uno, or playing Brawl Online, or playing any game, there will always be people like that.
2. It's already going on with Brawl, there are already people who are very insulting, very "elitist", etc to anyone who loses to them or to anyone who plays the game with items or stuff.
3. Also I'd like to point out that people are doing this for their own enjoyment and not to simply "put down anyone".
Actually, as far as I know, it shouldn't affect anything online or any other people unless Action Replay-like devices can work online but it's up to Nintendo to prevent AR from working online.
Though, I don't get what everyone hates about Snaking though. Snaking is not too hard to do, and it's really fun IMO, it adds some more depth to the game(IMO). Just holding the gas button and steering a little bit and occasionally shooting items isn't much fun for me.