Anything with perceived value will be targeted by abuse; if you want something not subjected to effort of abuse... make it worthless.
So far I've seen, trading cards are often ask by customers to developers. AAA publisher take their time into integrate trading card and other virtual goods (and hell know how AAA publisher are prone to do basically anything extra). New product, from the most insignificant shovelware to top tier AAA games use these features.
Valve lazy to remove it? I am the only one to notice the hell fest they make with virtualgoods in each sales event?