I generally play on a hard difficulty on a first playthrough, because most JRPGs have a tendency to be badly balanced on normal modes (not specific to Falcom). What I mean by that: normal modes rarely force you to use every feature of the gameplay systems at your disposal and try to master it. So if a game is easy enough without using a specific gameplay feature... I don't see any incentive to use it.

But the problem is that I'm generally a very indecisive person: if a game doesn't let you switch difficulties within a playthrough it takes days of online research before I decide what difficulty to pick. I have even started a new game few hours in, just because a game's difficulty was to easy or too hard.

So my motivation for harder modes is primarily: let the player chose any difficulty on the fly, so there are no worries of picking the wrong difficulty for the entire playthrough. BUT: one should only be able to lower the difficulty within a playthrough, but not switching back to Hard or Nightmare again so you wouldn't be able to abuse the difficulty selection to get past hard segments of the game and switch back.