As for the entire "why take 80% when you can have 100%", it's a trap. The argument actually has no meaning because it preemptively subtracts from the RDM and limits them to "80%" while ignoring what they gain. In effect it's saying a RDM without MP, without Dia, without Cure IV, sleeps, are all only "80%" and that the other nameless DD is given all it's options / ability's at 100%. RDM's sacrifice damage potential for utility. Back in the day's of "6-man" groups I would do...
It's hardly a trap. When your party is 5/6 people and it's time to fill that sixth spot with damage, it's not the time to be looking for emergency cures, a Dia your healer or 'lazy bard' could throw up, or even a secondary haster. They just want people who will concentrate on dishing hurt. Concentrating solely on that task, there's no bones about it, RDM fell behind due to native characteristics and gear options.

So, while Abyssea has changed EXPing dynamics from 6 to a possible 18, that 5/6 dilemma still remains for smaller events like BCs. When hitting the search list for a DD, RDM won't be on the radar outside of Azure kills. If the job is supposed to be suitably versatile, it's failing in that task if it will never be invited for an aspect of that versatility. Haters gonna hate, claiming that's how it should be or nothing could be done without godmode, but that only shows their lack of thinking.