The shells are held down by latches on each side of the front and rear shell, fore and aft. That's a total of 8 latches.
Each latch is a mechanical device that has a certain amount of stress and shock loading as you travel down the highway. It is reasonable to assume that any one of these latches
could fail at any time (as any other mechanical device could).
Having said all that, my trailer is 12-years-old and to my knowledge the latches are the originals. I have never seen anything posted on this site about lathes breaking (that's not to say there haven't been any).
I had the exact same concern and have
considered carrying a spare latch. Although, there is a left, right upper and lower roof latch and they are all different. That would mean carrying 4 latches. The better idea would be an emergency strap because the incidents are so uncommon as to not be worth the expense of spare latches.
If a latch were to fail while you were on the road, I doubt you would even notice it until you stopped somewhere. It's not like the roof could suddenly open or anything. Therefore, an emergency strap seems like a reasonable precaution.
Sounds like the previous owner may have been a tad "Obsessive compulsive" 
.