I don't see robes directly effecting xp gain, but they can indirectly effect xp gain.
I have suggested BW Robes to people recently because they are the healer robes. A summoner is a healer for their summon.
I'm not saying don't DD with your summon in PvE. Heck, with TP4 or 5 you can even tank in robes if you really want to, especially robes that give a PDef bonus.
Why I changed my mind and went robes in C4 robes. In C3 I used DC Light, I recently switched to DC Robes. +7 run speed ('m around archers a lot), para resistance (I fight a necro zerg) and +15% cast speed, as well as the fact I already had a DC helm are my reasons for DC Robes:
1. Faster casting with out using a gimp mage weapon
2. As a warlock, I don't have a summon with VR so it's all on me to heal if I'm unbuffed. I frequently xp off stunners so I don't want to tank. I can't heal if I'm stunned. Summon deaths in pve are almost non-existant for me now. In light I'd ocasionally lose one during bad aggro, even with UD.
3. My opinion is that cast speed is king in pvp. I think you can get pretty close with an acumen weapon in light armor but I prefer the look of my DLE over any mage sword.
4. Pets do tons more damage then pre C4. Even if I pull with a curse and wack first the pet still pulls aggro with in a hit or two.
5. Larger MP pool so I can hunt harder stuff for longer with Kai.
I believe if I were playing a PS, I would use Majestic light for A grade. Stun is always a big problem for a DE.