Autofilling people just to make the MMR work in positional matchmaking is not acceptable.
What I mean is: on our team we have someone who queued as ADC/mid and got autofilled support. My duo queued as fill, and was given ADC. This is, presumably, because his ADC MMR is different than his support MMR.
But now that autofilled support asks to role swap, and my duo, who queued as fill, of course says "sure." And both players are punished for role swapping with potential harsher splashing or the complete removal of their positional ranks. This is not acceptable at all. Just let the queue wait longer and assign people their proper roles.