Originally Posted by Morello
It's interesting because you're right - the requirement to finish a game you start creates a lot more behavioral problems as a result. This design is important because it's a double-edged-sword; one thing it does do, and part of the unique appeal of MOBA (prediction: this will not be unique in 5 years) is the investment in success one gets when you can't leave.
I'm primarily an FPS player myself, and join and leave those as I please. I like to win, but the investment I have in trying to win (and the associated joy/sadness) is ramped up a ton in League - mostly because I'm committed to the game session. My actions will have consequences - something I find sorely lacking in modern AAA development.
League of Legends is designed to be fun, but first and foremost, it's designed to be engaging. Tradeoffs do have to occur to make that happen - but the fact that we take on player behavior as a problem for us to deal with gives us a unique freedom to do design like this.
Not only this, but this sort of "when you're in, you're in" nature that League matches have by nature (and a nature that is enforced by the LB system) creates a better sense of uniformity and competitive fortitude--it becomes a stronger team game, because not only do you spend 20-50 minutes building (or trying to build) a bond with your teammates to ensure a greater chance of victory, but you also improve at a faster pace because when you mess up, you're forced to clean up after your own mess because you're not going anywhere.
There isn't that "oh I made a mistake, time to leave this game and just jump into another one" mentality (which I'll admit became incredibly tempting in TF2--getting camped by a spy because I don't know how to place my sentry/dispenser combo in more than one place? just go to a different game!), which really helps the game come together as a whole and keeps the community from spiraling into chaos.
When people are held accountable for their actions, they tend to put a bit more effort into getting things right the first time!