I have never broken a cable nor a chain or a spoke. Doing a cross country trip you will not need to adjust your brakes - if you ride during the summer. I believe if you use some Loctite 243 thread locker your screws and bolts will never unwind unless you did not clean the threads before application.
Never need to adjust your brakes? I worn out brake pads that were new at the start in the middle of a long tour (TA) where there were quite a few mountain passes and I definitely needed to adjust them in between. On the Sierra Cascades route I definitely needed to adjust the brakes even though I only went from San Diego to Reno. On the flatter Southern Tier I think I managed to do the whole thing with no brake adjustments, but I wouldn't count on it even there.
I have gone coast to coast a couple times and done a bunch of other longish tours and I too have never broken a cable or chain, but I have used my chain tool to fix a companions bike on two occasions. Even for a weight weenie like myself I can't see leaving the multi-tool with a chain breaker at home. It weighs a about 3 ounces and fits all the bolts on my bike as well as having a chain breaker.
Cables on the other hand I can see leaving behind. Unlike broken chains, broken cables won't leave you walking and breakage is unlikely if you keep an eye on them.
My Unior cassette cracker weighs less than an ounce so while I can see not taking one, there isn't much to save there.