AutoFill decreases the QUALITY of competition in Ranked games
Since the solo/duo queue system has changed, many people have taken up mastering two main roles. While AutoFill does shorten queue times, it completely ruins the purpose of why anyone would queue for a ranked game. The quality of a ranked game drops severely whenever ANYONE on both teams gets autofilled into a role that they are completely unfamiliar with.
Now you could argue: "Just learn to play all five roles". Put this into perspective. When someone has reached say Plat++ on certain roles that they have been mastering over a season or two, it's unreasonable to expect them to somehow play at the same level they were playing at on other roles. In many competitive games, it's not uncommon to see people master certain aspects of the game instead of being a jack of all trades. The argument that everyone just needs to learn five roles may have been valid 3-4 seasons ago when everyone just had pick order, but it's undeniable that many people in higher elos have developed mastery over two main roles and when forced into a completely unfamiliar role, the quality of the game drops severely.
The people who have obtained their rank from playing their chosen roles are there because they can play those roles. Many people simply don't understand that you can't just put someone into an unfamiliar role in a RANKED game and expect it to be just fine. There is a reason why many people play ranked games. In order to compete with other players who have also mastered the roles they have chosen to play. You cannot expect an autofilled support/adc/jungle to compete on the same level as a main of that role.
Not only does autofill make a team prone to tilting, it just completely defeats the point of why anyone would queue for a ranked game in the first place. Personally, I get my role most of the time so it's not too much of an issue for me. But I honestly feel bad for my lane partner when I get autofilled to bot lane and I'm sure many other people would also feel bad for their team mates as well when forced to play a role they are completely unfamiliar with. For the quality of competition, please either remove autofill or flex queue. tl;dr
People play normal games in order to practice unfamiliar roles or new things.
Ranked games are there however to test players against others of similar skill. Autofill completely defeats this purpose.