At what point does waiting to buy a ticket stop being reasonable?

[GatwickDepress]

And it's a shame it's being introduced on a TOC-by-TOC basis with no national planning. Most passengers don't really care which TOC they're travelling with so the artificial boundries on such a card are an annoyance.

Very much so. Look at Southern's The Key map along the East Coastway - thanks to Southeastern operating St Leonards Warrior Square and Hastings, it's not valid beyond Bexhill, so absolutely useless to me.