It is just ethic between leavers and non leavers.
It really depends on your sportsmanship, same goes to footballs, any other games in the world.
But for some reasons, I really hate to see leavers. Especially when you are putting "some" efforts into farming to a certain level; e.g: opponent keeps harrass you until you couldn't farm a single creeps; yet you maintain yourself and in the end, you get a slight chance to win back. And out of sudden, the opponent starts to leave. It really piss you off because the game hasn't show the winner/loser. Yet people just leave without any reasons; or simply because they hate to lose.

For me, win/lose is still achievement; at least you get to see something new during the progress in the game; even if you lose, you get to learn why you couldn't win or more or less, the tense when striking hard on to opponent yet lose. It's the progress that really important; but I know, typical Malaysians have the mindset :

If I can't f***ing win this game, why do I continue to die or feeding to the opponent. I might just leave and start a brand new game; a game that I could win.

Well, just my 2 cents and I enjoy DotA; not a Dotard but still I really hate to see people without sportsmanship.

Winner never quits, only losers quit and whine.