Quote from here:
http://na.finalfantasyxiv.com/lodest...46f445f1e93802
Morale
All PvP-related gear will include the morale parameter. In level 50 arena matches, the morale parameter will affect the amount of damage taken from other players, effectively replacing physical and magical defense.
If I read it right, you have to go with PvP gears or your phisical/magial defense stats will be zero in Lv50 PvP. There are only 3 sets of PvP gears and the lv70/90 request PvP points so everyone will just looks the same in lv50 PvP (except dye of course).