I've been working on AEGs for about six years now and not once have I seen a need for a fuse. Typically they die after a part is broken, for no apparent reason, or they don't even blow and you're still back at square one.
As for the car analogy, I fail to see your point. Now if I was going into the field without bringing a spare AEG/cleaning rod/etc. on hand in case my primary went down, then yes I could see how the tire analogy might apply.
Now if you're running some sort of expensive MOSFET or other electronic components in your AEG, then yes, I could see the need. Beyond that, no I refuse to waste money on them.