Would be nice, but I'm not familiar with how the European carriers handle it. Does the middle seat just get "booked" as if the person got 2 seats?

I don't know if I see F9 doing it, at least not as a Summit perk because that's one more STRETCH seat they won't be earning revenue on. Maybe if they hiked the price on Classic Plus (I kinda wish they could rebrand it business and go after FL's customers who are ticked they are loosing FL's business class) and offered it as a benefit.