What do you know, all it took for me to purchase games on demand was a good old-fashioned sale! A proper one at that, even if it's not quite Steam-level madness. I went from no GoD titles owned to having Tomb Raider, Witcher 2, Left 4 Dead 2 and X-Com. Spent a bunch of money and feel like I made out like a bandit!
I think the sale they did a couple of months ago was better overall. They had so many games on sale