Some people do compete for speed or score in single player games. Arcade games have always had scoreboards, modern "arcade-style" games have online ones, and a community can turn any solitary activity into a competitive one:
Speedrunners are an exceptional case, but I think everyone gets a little annoyed when they look at a leaderboard and all the top players have scores of UINT_MAX or times of 0 seconds.
Obviously cheaters will find a way regardless of whether you give them the source code or not, I'm just saying dfc's concern is not totally ridiculous.
I DO get annoyed when I see those scores, but in a lot of cases even having a leaderboard is just something that was introduced in the game just to be more "social" and less because it makes sense in that specific game.
And yes, it's not ridiculous, on the contrary, it's perfectly understandable.
Of course, these kinds of questions depend a lot on the game in question, and I think they don't have a definitive answer :)
oh don't get me wrong, i love speedrunning and trickjumping competitions, except that they always should require the whole replay - and even then you can't be sure if the whole thing was or wasn't TASsed.
Of course, in multi-player competitive games anti-cheating is a pretty big concern, because it works against the purpose of the game: a competition with well defined rules and conditions.
If the core of the game is single-player/non-competitive, why should we be so worried about cheating?
If the player has fun, it's a nice feature! :D