Now for all of your theoryizing that RNK/RUN will somehow get -95%MDT,
There is an overall cap of -87.5% damage taken, including sources that bypass the other caps.
Shell V is Increases magic defense by 62/256, 24.21%
Shellra V at 5/5 is 70/256, 27.34
The soft cap on MDT is 50%, Aegis goes beyond the MDT soft cap like Burtang goes beyond the soft cap for PDT.
Thus a 95 Aegis (piss easy to get now btw) only needs 42.5% MDT to hit hard cap. 24.21 ~ 27.34 of that comes from Shell V leaving 15.16 ~ 18.29 to get from DT/MDT gear.
Seriously, I have more then that on my "ohh sh!t" gear sets for WAR / SAM / DRK / BLU / RDM.
Heck Shell IV is 21.87% MDT, a PLD can hit soft cap MDT without needing a Shell V / -Ra V.
That is what I mean by Aegis already being our "magic damage tank", mechanically it's not feasibly to get better then it due to the hard cap. MDEF only functions on things with MAB, and then only as a fraction. MDT is a direct reduction and is typically a better way to go about reducing magic damage. Although lets be serious it's not magic nukes that wipes alliances, (except meteor) its 15~20 foot aoes that smash anyone who isn't a PLD.
Now if SE is thinking to create a melee that deals damage but is resistant to magic damage, then there might be a place for that. If their resilient enough to not be wiped like other melees, and can deal sufficient damage to be worth not bringing another melee, then they might be useful.