Carrying spares is a comfort. I have carried a spare serpentine belt for 25 years. The only time I ever had need of it, I was miles from nowhere in Nevada and it saved me a huge hassle broke down in the desert.
Usually being prepared means you won't need it. Seriously. The best way to ensure something breaks is to not be prepared. Last trip I turned around and came back 15 miles because I remembered I had forgotten the spare for my boat trailer. I went back and forth in my mind then said shit with it. If I don't bring it I will have a dam flat and have to wait an hour for someone to come with it so I may as well take 20 minutes and go back. I had live bait so I strain to get to the water while they're alive. I keep them in a drum with an aerator.
If you are crossing the desert it would be wise to carry plenty of water also. Lots of people have died of dehydration when they got stranded in a desert.
