This is a great way to prevent RDM from being invited to anything, ever - particularly in a game like this where the roles are so stratified. While the RDM in your example can fill any role, people will ALWAYS want a specialized instance of that role. Ask yourself: You're building a party, and you have the option of choosing a good DPS or a below-average DPS. Which will you pick? Sure, maybe the RDM COULD play as a below-average tank or healer instead, but right now you want a DPS. So, which will it be: good DPS, or below-average?
FFXI had a similar problem with RDM in the beginning. It could nuke, but not as well as a BLM. It could heal, but not as well as a WHM. It could fight with a sword, but its stats meant that it did far less damage than a dedicated DPS. As folks figured out ways to optimize The Grind (and in an MMO, optimizing The Grind will always become a priority, no matter what The Grind happens to be), RDM got left behind. To compensate, SE made RDM a specialist in its own right, making it the best enfeebler in the game, giving it some unique debuffs, and also gave it support abilities including one that was all-but indispensable: Refresh (restores target's MP over time - in FFXI, only SMN had native MP regeneration, and even SMN's was very weak).
So, the only way they were able to make a jack-of-all-trades, master of none usable was to make it a jack-of-all-trades, master of ONE. If RDM in FFXIV doesn't have a role at which it excels and is defined by, it might as well not be in the game at all.