when we say the best stun GCD is well used, pld lock out any player if other dps choose to be coordinate with this PLD it can mean instant dead to the player
by exemple you focus a healer you stun, silence, stun, an stun again with all the dps behind you burst the same target, this healer can be down without be able to do anything.
And also
cover (you may usualy use it on healer in difficulty, you don't just take the damage, you
prevent for some times you healer to be decast and he is able to cast free till the end of the cover)
and the 25% defense party -wide
and pld is able to do good damage anyway... he have some burst skill and dot, with dps accesories he can deal good damage, it's not as high than the mrd can do by bursting but mrd don't have the best stun GCD, cover, can't silence
(pvp is also about how good your group is and how them will coordinate, and it's true you need to be coordinate to take advantage of the control in pvp)
