At this point, the US military has determined that having paratroop units available is a useful capability in war. Arguing whether that tool is still useful is completely valid, and I would hope goes for anything: strategies, tactics, personnel, armaments, vehicles, equipment, etc.
However, once decided that a particular tool is necessary, it seems to me unquestionable that they should be kept in a ready, prepared, capable state. I suppose that too, can be debated, insofar as what constitutes "ready, prepared, capable," but if we've decided that our military needs people to jump out of planes, I would hope it goes without question that those people need to practice jumping out of planes.
shorter: training and necessity of a unit seem to be orthogonal concepts