Working as intended - it's not meant to be the primary means of levelling, but a carrot for running old content so that new players who need to do that dungeon/trial are guaranteed players queuing via Duty Finder. The once a day bonus is to stop players exploiting it.

There are plenty of other means for levelling - Roulettes were never intended to be run over and over again for EXP chains, it's as simple as that.