It's Not About Being Unable to Play Two Roles
It's that because someone who mains one role is forced to fill, no one gets their main role.
To put into perspective, I played a few games queued with friends. Out of 4 games I got my role once. One time. The rest were fill. The queue already assigns your role. So basically all that can be inferred is its their main role.
/Which isn't true/
I saw a post complaining about people still writing ADC pref mid pref etc. And someone immediately said report them. Just because someone has a preferred role doesn't validate reporting them. I was queued with my friends and I got top, my worst role. He got jungle, his worst role. So I told him let's switch. Only me and him have knowledge on what roles we can do so this worked. (But shows you don't get your main role regardless)
If I was in solo q alone. If I asked to do this. Someone would dodge or I guess I'm reported for wanting a role I didn't randomly get. See I don't want to play ranked knowing some of us might not be in our preferred roles. If everyone on my team had a fill secondary we can have a
jungle main ADC mid main support top main jungle etc etc.
You can check my rank and take my opinion for a grain of salt. But I'd rather have games where everyone is at their best than subpar. I can play any role. But I still have a main. It's not about absolutely being Unable go play a role but teams made unoptimised.
I'll never know what roles my teammates main if we are just a team lumped of 5 people who wrote fill.