That would only be a benefit of one is in the end going to accumulate enough avios to use them. E.g., an infrequent traveller with 25k avios wouldn't benefit by forking out the cash and being left with 36k avios.
Another point is that the realistic benefit of an avios is only what you would otherwise spend for. The 3p per avios value one gets for an F redemption, is worth a lot less if that person would never pay for F. Raffles makes this point very convincingly in one of his posts.