I'm pro-autofill, but I think it needs a bit of work
There needs to be a check after a game is found to make sure everyone's in the right role, and shuffle them around if they're not. To illustrate the problem: I queued up for a match and one was found, and I was assigned top. When it came time for the support to lock in their champion, they declared that they were autofilled support and didn't actually own any support champions. Then they dodged. Now here's the thing: I queued up as support/fill. I think the fact that people are getting autofilled into other peoples primary roles means that the system isn't working as well as it could.