Riot can't systematically give less LP/MMR loss to one team and full LP/MMR gain to the other. It'll break the Ranked ladder over time because of massive LP inflation.
They could give reduced gain and loss in matches with a leaver. However, this would be unfair to apply mid-match in some cases. For instance, let's say my team is winning really hard, and someone on the other team ragequits because their team won't surrender. It would not be fair to give the winning team less LP/MMR gain in that case. It's really hard to tell those apart consistently from someone just having technical problems. It also offers an incentive to troll or shame your teammates into quitting, since then you'd get reduced LP loss!
It might be reasonable to apply this in cases where someone never connects to the match at all. DOTA2 simply doesn't count such games in terms of their Ranked stats, for instance. Riot has said they're looking into doing something in this case, such as replacing that player and restarting the match, or maybe pausing the match for a couple minutes to give that player a chance to reconnect. Maybe we'll see something for Season 5.
While this is definitely frustrating, in the long run it happens to everyone. In fact, if you never have connection issues, you'll win more matches then you lose -- each match there are 4 players on your team who might DC, and 5 on the enemy team.