It's neither, it's a case of a stubborn playerbase who expects that the user HAS to have these things. Given that RUN only has a single ergon and most do not have it, yet it tanks modestly well, it's not a NEED. Paladin has it a little rougher because Aegis is a large boost past the MDT cap, so people absolutely expect it to mitigate the damage and reduce how much support is needed. That's just a playerbase thing; I imagine a Paladin with a Priwen can do a good job, just gear for MDT and deal with a 50% cap instead of like a what, 90% one? (Not sure where it caps for MDT II, pardon). Ochain is a little more arguable given its block rate, but Priwen with Reprisal up is supposed to be better, to my knowledge.

Either way, it's less than it's "needed" but more that players expect it, because they want to go with the absolute best safety net. It's the reason that melees are generally shunned for a MB strategy now; less support needed and "safer" because there's less people in the front lines.