Should you bring a back up camera body when you travel? That depends. Have you ever dropped a camera in a river? Or off a cliff? Then for you, for sure, the answer is Yes. For the rest of you, I know it’s a pain in the butt but the day you lose/drop/break/drown your camera, you’ll thank me for this advice.
For a while, I carried a Rebel T3i as the back up for my 70D. Now I carry the 70D as a back up for my 5DMIII. I’ve also carried a point and shoot as a back up. In a pinch, your iPhone works too - even better if you add a clip on lens accessory. A mirrorless system is a great choice too, if you have one. The key to bringing along the right back up gear is to be comfortable using it, and to bring everything you need. For me, the problem with bringing the T3i or a point and shoot is that I have to bring different batteries and chargers. I never want to give luggage space to stuff I might need, but probably won’t. The 70D and 5DMIII use the same so I can grab 1 charger and 3 batteries and call it good. The same issue goes for the mirrorless. You’ll need to pack both batteries and a charger as well as lenses. In some cases, you can use an adaptor so that your primary lenses fit your mirrorless. Or, you can attach a nifty fifty to the mirrorless, and go old school for the rest of your trip.
Chances are, after reading this, you’ll bring your old camera with you the next time you travel. Chances are that you won’t lose/drop/break/drown your primary camera and eventually, you’ll stop the madness and leave that extra weight at home. That’s when karma strikes. I know. I went for years without harming a camera - to the Dalmation Coast, to Tuscany, to Rome, to the Amalfi Coast, to New Mexico, to Tennessee and on and on. My gear survived Iceland without a scratch. So when I went to the Adirondacks, I left my back up at home. Halfway through my trip, I dropped my camera in the river. I had nothing but my iPhone 5C (no lens clip) for the next 3 days. I WAS ON A PHOTO WORKSHOP.
Bring a back up.