I usually finds some use for them. In the era of tanks, pre Mech inf, my tank city assaultforce usually rolls away from the defending inf. In those cases a squad of Paratroopers that drops in can hold the captured city until the inf arrives while the tankforce rolls on to the next city.
I have used them for invading islands if the distant is short enough. Can be handy if you lack transportcapacity to get that beachhead so your airports can start shipping in troops.
You could in some cases use them paradrop in to pillage some strategic resource that's hard to get with other means, but in this case I find Spies, Bombers/fighters better options.
So, with the right circumstances and strategy, there's some use to the paratrooper.