I thought that until I started reading more posts on this site
You can use them for straight knitting though! I'm doing it with a shawl at the moment. All you do is cast on as normal and start knitting with the free needle, then when you get to the end just turn the knitting around and start again like you do with ordinary needles. It's best if you try and forget about the cable when you start if you get confused like me
I hope I've explained it ok for you!