Cheating online is best stamped out as fast as possible - if a particular server or game doesn't ban cheaters I won't go back.
I completely agree.
But I also don't really get why you would cheat in a single player game at all. If it is a game you can play SP and then MP online, cheating at SP would prevent you learning enough to beat online opponents.
That's an odd argument. First of all, you seem to be thinking of a very specific type of game: the kind that HAS online multiplayer. Not everything does, and I don't think I play ANY games that do. That said, wouldn't someone know they weren't properly preparing themselves? I don't think anyone practices for bicycle races by riding a moped everywhere, and is then surprised when they trail on race day. Why would this be different?
To me it's a part of my completion habit - I want to be able to complete the entire game myself. This means there are still some of the advanced challenges on Portal I haven't yet done, but I won't cheat to do them - that would defeat the entire purpose.
I get that view, and I share it on SOME things. In recent history (the past few years), I've unlocked every single part of Mario Kart Wii and Plants vs Zombies. I'll complete a game if I'm having fun while I'm being challenged, but not if I'm getting beaten down.

And surely you must condone cheating on replays to get more entertainment value out of a game?
