Why mastery is needed in Ranked
Players don't know their champion I'm sure everyone has dealt with this problem before; your teammate whether on good or bad terms decides to pick a champion randomly in your high valued game. Because every match is very valuable to us league of legends players :) This also gives us assurance that they really are a "one-trick" rather than a troll.
Spur of the moment picks, and troll picking Many games are decided through picks and bans, but so is champion mastery. Like I said about assurance of this player picking a champion that you don't see very much. Now instead of someone picking miss fortune support or something weird, they'll have high mastery on that champion and it will seem more viable and PREVENT CHAMP SELECT TILTING
Anti smurphing and fewer new players in RANKED Everyone hates smurphing players, except for the smurph players themselves. cough Apdo This would also indirectly get rid of players making boosting accounts, well, aid against it.
Priority punishments can go to lower mastery players This system won't COMPLETELY fix the troll players, but it will funnel them, refine the game, and overall give us better matchmaking quality.