Did you know that ethernet cables can carry HDMI video signals? When two ethernet cables are paired with an adapter, you can run an HDMI signal over long distances at a fraction of the cost of an HDMI cable.
While we would still recommend a traditional HDMI cable for your primary use, if you just stick one HDMI cable in the wall, you're out of luck if it goes bad. It can get extremely expensive to buy two, just to have one as a backup in case anything goes wrong with your initial wire. Ethernet is pretty cheap, especially when compared to HDMI cables, especially at longer lengths. For $150 you can get 1,000 feet of Cat 6STP ethernet cable. That's 15 cents per foot. A 50 foot 24AWG HDMI cable, even at Monoprice is $40. That's 80 cents per foot.
For a cable that's just a backup and you might not ever need, ethernet makes its case. If anything, that sturdy/cheap cable makes a great wire pull in case you need to thread another cable through your walls. So next time you're renovating and pre-wiring your home for video, don't forget to drop a pair of ethernet cables in with the HDMI for an inexpensive and easy backup solution.