Stealing Pokemon is a classic example.
There are several places in the games where the game benefits you for having guys from lots of different cartridges. So it's worth it to take those guys. Besides, sometimes someone has one of the rare promo ones...even if the person who used the old save cheated to get the promo one, I know some people that feel like: well, it wasn't ME that cheated, so it's OK. But they wouldn't use the Action Replay themselves. Weird, huh?
But for other games: I never use someone else's save game. I rarely buy used games, for one, and when I do - especially if the game has a story - it doesn't make sense to play through. Of course, some of my OWN save games where I've put down a game for a year, maybe two, feel like someone else's save...because I've forgotten everything that happened.