I did not ever say they were better. Unlike most of the closed minded people on these forums, I prefer find all the ways to make all my class effective in as many ways as possible. Truthfully, if you build your PGL correctly, they will take less damage than a MRD and Especially a GLA right now. They have a naturally higher evasion and First of Earth raises their defense to more than compensate for the lack of armour. If geared correctly, they can use their evasion and parry to do a constant stream of Reactive damage, while avoiding damage entirely and have the ability to repeatedly stun the enemy mitigating a large amount of DMG.
I can't definitively say PGL are better tanks than GLA and MRD, but I can definitively say that GLA and MRD are not better tanks than PGL.
I supose MRD has the advantage over GLA and PGL because it take little skill to play as tank where as GLA takes moderate skill and PGL takes quite a bit.