This has been the number one problem with the game for me for years now and the thing bothering me more than any other by far.
So basically 9 people join a ranked game determined to win and give their best where 1 guy ( usually more )
-has just bought a champion and want to try it for fun ON RANKED
-knows he has no idea how to play champion X he picked and still picks it
Now the problem here is that this guy with no experience or skill with the champion he took,will either die uncotrolably or just won't be of any use for his team and will not do what the champion he took is made to do.
There you see how 1 player ruins the game of 4 ( 5 percent manages to win but that's irrelavent ).
Now I'm not saying the system is flawd because the chances of that player or player being in your or enemies team is the same so you'll most likely get the same amount of those players with or against you.
***However he still destroys the quality of a game that people are giving time to play because of his selfishness***
Solution is a rather simple 5 normal games trial (initially I put on 25 but everyone seems to focus on the number and not on the idea,the number is always changable ) with a champion before using it on ranked and NO,you can't learn to play a champion or come even close to mastering it in 5 games but it gives you some glymphs of how he is played.
Lastly just going to say that if you played a champion once and did good in a game doesn't mean you know how to play the champion,not at all.