It's still safe with people you don't even know, because you trade cards by doing an offer : cards in someone's inventory for cards in your inventory, and everything is Steam-managed. Everyone gets the other guy's cards at the same time. There is even a period where you can't use your cards just in case there is something even with the security.
If there is enough people interested, we could make a new topic about it, and list the games we'd like to make a badge for, and the ones we don't care about, and then let's go for the trades.
As for the interest of all that, mostly trading i'd say, people simply sell their cards and buy games, other use the whole panel of crypto-money that it in fact is : cards, gems, then CSGO or TF2 keys, etc... And if you don't trade, it just has the purpose of adding stuff to your account : background images, smileys, etc... Not very useful in the end, but nice.