I've been thinking about why they keep that type of report as an option. Riot has openly stated that it doesn't get anyone punished in any way. So why keep it in there? My guess is, since the more you report the less valuable your reports are, Riot wants quality reports which means they want to remove the players who spam reports. Having an option like unskilled which is very appealing to report someone for after a bad game takes care of that. If you report someone for unskilled in 50% of your games, soon none of your reports will mean anything. Riot wants people to report for legitimate reasons, but there will always be those who just want to report someone because they feel like someone on the team was just bad and without the unskilled option Riot would receive a lot more reports for negative attitude or intentional feeding. Having the unskilled option is pointless but at the same time genious, takes care of A LOT of false reports.
Just 2 of my 10 cents.