You're going to end up sitting next to a stranger anyway if you're flying alone. Nobody says you have to become friends, but I wouldn't mind having in common with my seatmate that we're both the kind of people who don't take the standard option at face value.
If anything, booking together with a stranger would allow you to leave an empty seat between you which is less likely to be filled than if you leave two empty seats next to you.
And the service could even be set up to give you some choice over the stranger (at the expense of less matches) like setting a maximum weight.
Any time I do select a seat in advance, I try to pick my window seat where someone already has the aisle one. It works surprisingly often to get an empty seat in the row!
I've always thought there's a difference between who you book with and who's on your itinerary. Very rarely do I say I'm traveling with anyone unless we're staying in the same room. I guess these fares do specifically state that, but I have a very hard time imagining anyone at the gate would care, they're typically doing the bare minimum as they should.
A company can figure out the premium and just average it out across the pax who book thru them. Further they could risk-manage no-shows or other bad behaviour based on ratings and feedback. It's just wasting everybody's time to go thru intermediaries.
Because they may change their plans. They could be a no-show (which will affect your return flight). They could call and change the flight without your knowledge. They could add extras to the trip and charge it to your card.
People are flaky, and being on the same itinerary with the same PNR as someone else means your trip is in their hands.
Some sort of service that sat on top of bookings would have its own set of terms and conditions that you agree to, which would at least disincentivize them from acting against your interest.
