Personally, I dont ever buy single player only games, I just rent them or borrow them off a friend since I am one of those gaming addicts that picks up a game and cant put it down until I've finished it, my personal record was finishing FO3 in one sitting, only had to stay up 3 days... there was alot of redbulls... and nodoz... and it resulted in a rather annoyed gf who was both pissed and surprised that I pretty much didnt move from the couch for half a week...
But for any game with a MP component, I tend to rush through the SP mode in easy mode for the story (without worrying about achievements and bonuses) and to basically get a feel for the game and it's controls. This usually only takes up to 4 hours then I jump straight into MP to boost my K/D. It works pretty well too since most people who jump straight into the MP have to adjust to the game's controls online, whereas I've already adjusted through the SP so I can generally get a really good K/D (usually 5+) during the first week of a game's release.
As for trash talking little kids, well thats why I never bother with headsets unless I'm playing with people I know or if the team is working together really well (which is generally an indicator the team isnt full of little kids).
The main reason I prefer MP though is because I'm rather competitive (very actually) and I'm borderline OCD when it comes to just about anything where I am competing against other people, and this doesnt only apply to gaming! My addiction to caffeine has also given me super fast reflex cuz I'm pretty much always on a caffeine high, it's actually quite trippy when the game seems to be running in slow mo because you are that hyper =]
And for all the casual gamers that say MP games are unfair to casual gamers because of the lvl requirements for unlocking stuff, the reason most casual gamers get slaughtered online is not because of the level difference, but more because of the skill difference. Someone who has been playing 100+hours will most likely be better at the game than someone who has only played 10 hours! Especially since in most MP games, the basic weapons are generally way overpowered anyways. CoD games are a perfect example of this, the starting weapons are made more powerful so low lvls cant stand shoulder to shoulder with higher lvls, but then experienced players abuse this and you end up with lvl70s running around with a M4/Famas slaughtering the lower lvls who havent yet figured out how crap the higher lvl guns are!
Downside is, I get bored of games VERY fast, most games only last me about a month before it starts gathering dust on the shelves. Suppose it's also a good thing though, because otherwise I'd be stuck infront of a computer/console constantly... Atleast my uber short attention span allows me to go do other stuff...
Developers who develop just multi player games I think are too lazy to develop more real challenging AI.
With the limits on modern technology, no current age AI can even come close to a human mind at all. With shooters it isnt that much of an issue since devs can just give the AI accuracy bonuses to make the game harder. But the weakest of AI can be felt hardest in the strategy game sector. Even though most games give the highest lvl AIs resource and build speed bonuses, a good human player will still beat them even when seriously outnumbered. eg in C&C3, I can single handedly take on 3-4 brutal AIs depending on what faction im using, and the furthest i've ever gotten online are the preliminaries for tournaments (the first game after you qualify for tournament entry). the top players online have been seen decimating even 7 brutal AIs at a time!
Im waiting to see how strong the SC2 AIs are though, hopefully they'll be more of a challenge against metagame strategies... most current AIs only work against players that camp. since the majority of online strategies revolve around speed, AIs never stand a chance against someone using metagame strategies....