Still, too many players leave ranked matches due to a lack of confidence, huge doses of embarrassment, and absolute rage when they are scored on. This was supposed to be fixed in the most recent update, however a 5 minute ban from playing is nothing and won't correct the player for his/her mistake.
I suggest increasing the ban time to at least 20 minutes, and the time would only increase if the player consecutively leaves matches.
Players disconnecting is very common, and that is why a reconnect option is available. It is very unlikely a player disconnects right BEFORE the game ends, therefore it is not worth trying to balance out occurrences like that.
Players must finish the game before they can leave, as it is only fair to their teammates.
Should you disagree, explain why. If you believe you should have the right to leave in the middle of the match without suffering consequences then you need to go play quick-match or get on a different game entirely.