Is it safe to put (3)DS Game Cards in your pocket, or will they break? (I mean, without any sort of protection)
It would depend on the size of the pocket, what else is in it, and how much stress you put on it with everyday movement. Loose pockets will probably be fine, as long as you don't sit on them. Tight ones, though, might break it, or dig into you.– FrankNov 28, 2013 at 21:22
As fbueckert commented, there are a few factors with the pocket itself. However, as quoted in this article, Nintendo's President and CEO Satoru Iwata has had Nintendo design their handhelds with possible damage from a child's carelessness in mind, so the cards are designed to be durable. In my experience, my DSi has suffered more from being in my pocket than my games.
As long as the games aren't getting exposed to very high temperatures, the metallic contacts aren't getting damaged, large amounts of pressure aren't being applied, and they aren't getting drenched in liquids, your games should be fine. It doesn't hurt to keep them in a case though, as they are designed to add another layer of protection, in case of those freak accidents.
1And whatever you do, don't forget that it's in your pocket and let it go through the laundry.– BrianDec 3, 2013 at 18:09