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!
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.