So the answer might be to save the priority boarding fee and spend it on choosing our aisle seats. Simples!

So presumably, if you don't pay the extra by specifying your seat, then you will be allocated a middle seat every time?

At least with some of the airlines that charge for extra-legroom seats, if you don't want to pay the extra you can still opt for an aisle or window seat at check-in.

With AB you are allocated seats when checking-in online but if you don't like the ones they have selected it is possible to change them FOC. Unfortunately FR won't be that flexible.

EDIT: Of course, AB is not a cheap airline these days. Far from it.

Flybe will allocate you a seat on check in but if you ask nicely you usually get your choice FOC.

A few times now I've been at the Monarch check in desk and when they ask if I prefer a window or aisle seat I politely ask for a window seat towards the front if possible. The check in person then looks at their monitor and shakes their head, with a good bit of teeth sucking and comes out with something like "Sorry sir, the flight's very full today and there are only a couple of window seats at the back of the aircraft".
So I settle for whatever I can get, then when the plane finally departs it's only half full.
I honestly think that most airline staff have nothing but contempt for their passengers.