Scary thought...

For me the answer is simple: Never.

I wouldn't buy a game just for those extras. But I don't have to really worry about this because I already own most games that will end up getting badges.

I'm still debating if I plan on selling my cards or not.

I would never use a background from a game I didn't like regardless of it's cosmetic appeal.

I feel guilty. Joining the HL2 and Portal 2 giveaways just for the cards

I would never buy an game only for those extras, but im thinking about buying Dont Starve (people say it is good) with the cash i got from selling my cards, play, have fun AND re-sell the cards, to use it on another game.
On the other hand I think i should get at least 1 background. Cant choose between Half Life or TF2. SO EPIC.