The Rune Fencer in FFXI played like the RDM of past games.
The RDM in FFXI(while soloing) played like the Rune Fencer of past games.

FFXI gave them En- spells to boost their melee damage, but those spells actually belonged to Rune Fencers(in FFV anyways).

RDM as tanks makes the most sense because that is the only role that will allow them to make full use of black magic, white magic, and melee.