The 10amp fuse you mention would be suitably sized for the application. It is done so with knowledge of the design in mind. If the heater only ever needed 5 amps this would be made to 5 amps, the key here being it is < 13. Thermal cutouts of course are nice and still also apply to my mention of protection of circuitry.
I am going to treat thermal cutouts separately as I feel that a different discussion than this particular form of fusing.
I completely agree that bodies such as UL require these appropriate fusings for safety, however when you select the fuse, you don't just by default throw it at 10. You size it based on your application. Thus, inherently, it is there to protect the circuit too.
You also must think that fusing often times really is 'only' to protect the circuit. Lets say I have a +24V rail that can only drive 500 mA. I'm going to fuse it for 500mA in the event someone drives a digital output on that rail too hard etc.. so my driver IC doesn't fry, aka, protecting the circuitry. This subsequently would also blow if someone shorted +24V to chassis (which is a situation that is completely safe, even if chassis wasn't grounded, assuming +24V isn't floating). Therefore your original statement of protecting people versus circuitry does not cover this case, and as such cannot be considered a rule of thumb and blanket statement. Whereas if you use appropriate measures to always protect your circuits, safety will inherently follow.
Think about it.
