some of that stuff is pretty expensive and will normally "fail with warning"...for example you can drive home on a bad alternator, but you really should bring an alternator/water pump belt cause that will bite you 10 times harder. Tools, fluids and lube, ratchet straps, tow strap, baler wire, stuff to get off the trail...then you can bring the more interesting stuff like bar of soap and plastic bags.
that might not be bad stuff to have on the shelf, but I really wouldn't want an alternator to fly up and hit me in the back of the head in a collision lol
Edit: you can go crazy with this and bring spare motor mounts and stuff...but if you already spent the money on the part, why let it go bad in the back of your truck, or worse yet, wait till you're stranded to replace the part in the wilderness? Replace said items now if you're really worried.
