RD, I've never had a plug lead come off, despite being on the road for 40+ years. I see the issue like this. The cap comes loose and theoretically could unscrew all the way. Trying to retrieve the terminal from the plug cap could be tricky at the side of the road unless I had some long-nosed pliers to hand. I also have a 'thing' about stuff coming loose whilst I'm riding/driving, and I enjoy finding different ways to fix things that shouldn't ordinarily need fixing. I have about 4 pairs of loose terminal plugs which I want to use with confidence. If you put all these things together, then my purpose becomes a bit clearer. I also see this as a fix rather than a patch up.
Chances are, I'll never really have a problem, but the terminal was half off after a 110m ride. The bike did vibrate a lot more when the condenser was on its way out and I had a lot of misfiring, but it's much smoother now.
One of the best things about the forum is picking people's brains about stuff that is important to them, but may seem a bit insignificant to others. In the grander scheme of things, you're right in saying that fixing loose terminal caps on spark plugs is not something to worry about. Having said that, the guys on the 'petrol head' forum I mentioned have had a lot of issues with this problem. I think it was a dirt bike/off road type of forum.