I think the point Swanhunter was making is that a fully flex F ticket allows you to travel on any flight on that route in F, provided there is seat availability. A redemption fare (i.e. the mileage amount) is only available on strictly limited services. So it is a bit fictitious to view a mileage ticket in the same way as a fully flexible F ticket.

An A-class ticket tends to be easier to get than a mileage redemption fare. Whilst fare rules are more prohibitive than a redemption fare, this has to be offset against the greater availability of A class tickets.