Bloodygoodgames: I'm always fascinated by the gullible when it comes to 'collecting', especially as things like Steam cards are, for the most part, completely worthless.
A clueless gullible person waves hello to you. If collecting something entertains me, does it really matter what it is I'm collecting? I could be collecting recipes, shoes, cars, butterflies, games, badges, alcohol. So please refrain from insulting me (and the rest of the people who collect). Some do it for a hobby, others due to an obsession. Bottom line is, collecting badges entertains me, thus I set a bit of money on the side for the cards.
Yes, it was a brilliant move on Steam's part (money from nothing), and as for the users, some enjoy it because they make a bit of money selling the cards, others enjoy it because they enjoy the collecting. But I've spent ~€5 for much worst entertainment than collecting badges, so I don't complain.