Of course, playing on-line have it's own perks, just that it's very rare to encounter pure awesomeness fun moments (they are dwarfed by the reasons i listed above).
Reasons on-line is awesome:
1. Playing games with buddies on the other side of the world, priceless.
2. You're playing with real gamers, AI is predictable, but not real players (again, you have to overcome the "stupidity" issues, which makes this reason a rare gem).
3. Love the game, but beat it many times... Need something new to overcome the "good game withdrawal syndrome?" Try multi-player, otherwise wait for a few years for the sequel.
4. Love to compete? Have a superiority complex? Rule the world by crushing other gamers on-line instead of your local tiny little group of friends (they might suck really bad and you probably don't know it). But please no cyber bullying.
5. Stimulate a real strategic battle/mission? You need to play on-line... To test your request for leadership, honor and glory. There's nothing more satisfying that a win from a properly planned, executed and cooperative game play.
6. Have your own ideas? Want to share your creativity with the world? Create your own levels and let your fellow gamers praise your creation, very satisfying yes? Well, can only happen on-line... Not all games can offer you this option, but it's part of the on-line feature.
7. Another reason for paying your internet bill