If the system detects a leaver on your team

TL;DR: That loss should not count towards your series. Plain and simple.

Here's an anecdote:

Today in ranked, I faced a team that started off doing pretty well for themselves. The enemy Xin Zhao got the better of me at level 3 and it resulted in a first blood for him and his team. Bot lane was doing well for themselves as well, with their Ashe and Blitzcrank keeping the pressure on our Kalista and Braum. When our Jarvan IV was en route to help me out vs. the Xin, he found their Amumu heading top as well. With the help of our Urgot, he chased down and killed Amumu in his jungle, which resulted in him leaving the game soon after. Eventually, the enemy team would lose by way of attrition. Unfortunately for both the Xin Zhao and the Ashe, they were both in the middle of their promotional series, and were handed a red 'X' for losing that game.

With that anecdote aside, it shouldn't matter if a team was "going to lose anyway" if a player on their team was gone long enough for the system to detect it. The other nine players were not allowed a fair game, which is, quite frankly, a travesty in a ranked environment.

In my point of view, the arguments regarding the abuse of such a system are no longer relevant. Lyte has announced an automated system that is soon coming out of its iterative stages that should work better to combat certain cases of toxicity and punish the offenders in as little as fifteen minutes after a game ends. Anyone who would potentially be harassed to the point of leaving a game is now further encouraged to weather the storm and report the offenders at the end of the game, with the promise of feedback on their report assuming the offender is guilty of said toxic behavior.

As for the people who would intentionally leave games to "save" someone's series from collapsing, we already have an automated system in place that detects and punishes that. If it can't detect cases of abuse when it's been running this long, I'd highly suggest that it be looked into or possibly overhauled.

The fact of the matter is, we're in the second half of the 2015 season and this is still a major issue that needs to be addressed sooner rather than later.

Once again, TL;DR: A loss due to a detected leaver should not count towards your series. Plain and simple.