It's a good idea to back up. I guess as photographers, we get comfy at times. I can't remember the external HDD brand name, but I used to use it every time when I traveled a lot last time.
Chances of SD card failing are slim. But they DO happen. It just needs to happen once to make you feel enough pain. So getting more cards is not the solution of a back up.
I think if you shop around those good shops selling camera accessories or memory cards, you might see such back up drives.