"fun" is different for everyone. some people have fun having an unfair advantage over the game.. and some people prefer the challenge of it all. thats why people will cheat in online games lol. once they can become invincible, they have fun making everyone mad...
a long time ago, i used cheat codes (not bugs) to become invincible in a single player campaign mode of a game, and it was fun just running through and looking at all the AI try to kill me lol. I got through the story pretty quickly..
edit: LOL.. didn't really respond to the Q.. as far as using bugs in game to take a shortcut, i personally find it boring. i know someone who will brag about how good hes doing in a game, and how he has no trouble completing the most difficult parts... in an MP game, he tells everyone about how high his rank is and how he's really good.. he told me that he will use bugs or cheats in single player.. and he mentioned to another person that he paid for his high level mp account.... THATS where it bothers me lol. go ahead and use those things, but be humble about it, and don't brag about how you succeeded faster than most people...