I hate Auto-fill too and WILL dodge if it happens to me and nobody is willing to give me the role I wanted.
The point of it is, the queue will seek players of similar skill levels when it matches you. And as the queue timer ticks on, the more lenient it gets in finding someone.
In times where there is a lack of people playing certain roles. The queue will eventually cast a wide enough net that the person it eventually finds could be significantly better or significantly worse than their team.
It's this kind of mismatch that auto-fill is an attempt to avoid, but what Riot doesn't seem to realize is that players are just BETTER at certain positions. A God-Tier Top could make a Shit-Tier Support and vice/versa.