Marksmen Should Not get Autofilled
They play the most crucial role in a team and the difference between a person substituting an ADC player and an actual ADC main is the distance between the sun and earth, from the way they move, to how they cs, trade and what-not, you can tell someone's not adept at the role in less than a minute of how they play the first 2 levels.
Other roles people could manage for sure, single 1v1/warding here and there,ganking etc. but a top lane player (let's say a riven/jax main) gets put in a 2v2 and suddenly he's on training wheels, making numerous mistakes, not taking the 2nd and potential third player into consideration is a big no-no in my opinion at least.
Granted autofill is obviously not wanted ever, but in some cases it's necessary in order for us to find and play at least a single game in an hour, then again some exceptions could be made for the most volatile role (i hope).
/rant, My thoughts