1. RDM damage isn't low, but it's not super great either. It's decent. But they have other tools that make them useful as well. Biggest flaw is their AoE damage is severely lacking and/or takes a long time to build up (getting Enhanced Moulinet takes a while)

2. RDM has insane utility, they get a useable Cure spell as a DPS and they have an instant Raise. Embolden is super nice, too.

3. "Useless" is hyperbole. It may not be "optimal", but people who strive for speed runs or whatever will always pick the most specialized/optimal DPS jobs. That doesn't mean the remainders are "worthless". Also, obviously not every job can be tweaked to be "optimal" because it depends on the party composition and the parameters of the battle itself.