The only reason i don't like Auto Fill.
Bad Supports.
Like, i know it's pretty hard to be bad at most support CHAMPS but the role itself can be pretty difficult and there are just as many things to track in a Duo Lane as a Solo Lane.
I'v played 8 ADC games in about a day or so, 7 out of those 8 games, i had a support that was auto filled. In those auto filled support games my sup would frequently be unable to lane properly, they would walk to far up and get chunked quickly, would run off to chase someone down while i'm being 2v1ed or wouldn't use their Heal/Shield ability(if they had one) on me correctly. This goes without saying but those same sups would frequently KS me, then die, now KSing is not that bad but then dying after is awful, because that means that not only did i not get the kill but they did, putting me behind in a lane where my Janna has kills and i don't, that's pretty bad.
This is the only thing that makes me hate Auto Fill, i'm pretty sure in High Elo it's not that bad since if your Diamond+ you should know how to play every role well enough to not be a feeder but in low ELO games getting auto filled cripples that team.
(Keep in mind that 80% of the player base is Low ELo.)