Though in a Paladin's defense - they are pretty much obligated to get an Aegis if they want to be able to effectively tank many of the higher-tier magic-based mobs. RUN isn't - and can get by without having to spend months and months farming in boring Dynamis or 100's of millions of gil buying one. So there is a point to be made there.
I might be more sympathetic to this if magical tanking wasn't supposed to be RUN's primary role, and if a RUN could compare to a PLD's physical mitigation without its Ergon weapon, which is much harder/longer/more expensive to make compared to an Aegis. Ochain is PLD's best physical defensive shield, but if you're not worrying about magic then you can probably get away with a Priwen with reprisal or something. Aettir gives PDTII-5, which is not much, and should have been 10. Dunna is 1/2 the potency of Idris, but Aettir is only 1/5 the potency of Epeolatry. Should be PDTII-10, but I digress.

All the suggestions about altering the functionality of runes are, I think, unrealistic for SE to implement at this stage. Personally, if we were to go down that road, I would have liked the Runes to provide resistance against the element of the rune itself and the element that the rune is ascendant to. This makes Lux and Tenebrae kind of redundant though except for enspell and lunge/swipe damage, which I suppose would be a legitimate reason to choose between the two, other than Vivacious Pulse with Tenebrae. This would double the number of your resistances without changing how runes work too much. However, like I said, I think changing the way runes work in even moderate ways is probably more work intensive than the devs want to bother with, especially with the last patch coming up very soon and the last chapter of RoV needing to be finished on schedule.

So, I hope the devs/representatives are reading this. Magic Evasion Bonus VI trait and a large, flat magic evasion or magic defense bonus or both (somewhere in the ballpark of +50 to +150) added to Foil are simple changes that would immensely improve Rune Fencer's ability to do the job for which it was designed without adversely affecting game balance. Let the magic tank be a magic tank.

P.S.- Oh and change Aettir to PDTII-10% and add Inquartata +2 to it. But I know you probably won't do this.