Oh I am not arguing that point ^^
I even said so here :
I am well aware of the math going on behind the mechanics (even though my personal "let's go for the worse so we'll never be disappointed" led me toward 2nd formula ^^).Now does it really matters ? Well, not that much. After all, you're gaining more from blocking than you're losing on the alternative (parrying) so overall your mitigation improves, and is better than the one of a WAR.
PLD does get a way better mitigation overall thanks to his shield, that's not even arguable. And WAR benefits more for every point gained in parry than PLD.
This is the point though. As tanks, we'll get a hell lot of physical hits, and some are really nice to block/parry (Death Sentence or Ravensbeak to quote the worst of them I witnessed at this time ^^) As a WAR, you'll want to stack a lot of parry for that, but parry is detrimental to the other side of a WAR. Every point in parry is a point lost in crit/deter. So you need to achieve a reasonable parry rate while not hurting too much your dps.Thus, having less Parry won't hurt a PLD as much as it can hurt a WAR.
PLD gets the same problem, but in a lesser way, as their primary goal is to nullify damage rather than to soak it by self heals or than to deal damages, etc. They can do it, but it's an extra. So stacking parry hurts less as the other stats have a lesser impact on their overall role. Starting from here, while every point in parry adds less to your mitigation than WAR, it is also hurting less your utility, and the balance is more easily made.
PLD have the advantage that having more or less parry won't hurt them as much as it can hurt a WAR
At least that's the way I understand it
TL;DR :
considering parry alone, WAR gets more than PLD for that stat. Once put into real action, PLD gets more benefits from it than WAR. For both though, balance is required not to detriment other important stats, even though PLD will suffer less from a poor balance than WAR in a MT gameplay optic. (Parry being totally BS in OT gameplay anyway xD )