Imagine loosing a bag with 5D, T2i, 16-35, 24-105, 70-200, 24L, MBP, iPad. Thats around $13k+ in gear. Ouch!
That's why you never take your eyes off your gear and that's why I used my Tilopa as a pillow on the trains in China and it stayed glued to my back up two mountains until we got to a decent hotel in Beijing that was secure Locks are key as well but they can sure be a PITA at airport security when you're unlocking all the compartments of your bag
I'd say for most people you could get by with a single body, wide angle zoom and something like a 24-70/24-105 and a single fast prime if you really need it. I thought for sure I'd use my 35L/85L more in China but even indoors my 17L gave me the shutter speeds I needed.