Are you talking about EM shielding or structural reinforcement?
If it's the latter, realize that Kapton (polyimide) is pretty fucking strong as long as it doesn't have cuts in it. Also if you look carefully you'll realize that many flat flex cables are bonded to another piece of flexible plastic for extra durability.
It's not bad design to use these cables for flexible joints. That's their design purpose. As long as they are properly strain-relieved on both ends and are protected from getting pinched they will flex and carry signals for a long time.