This question popped frequently in /r/guildwars2: “Should people doing SPvP earn some XP?”
My instance on this was the more pragmatic possible: “I don’t think they should mix a PvE ‘currency’ in a PvP environment.”
If I wanted to expand, I’d go with a “There should be a careful balance in this. If there is enough XP in SPvP, people won’t bother with the PvE and would only do PvP till they reached level 80, enable a bunch of skills and all traits and then roam through the low level zones wreaking havok (yes, you’re downleveled, but the access to more powerful skills and all the traits do make a difference. On the other hand, if the XP is below the PvE experience, people will ask why it’s there in the first place. So, without mixing those two, ArenaNet avoids a bunch of balancing problems.”
But recently I went into some weird thoughts about PvE currency in PvP:
You can reach level 80 if you go max level with all crafting professions. It would be an alternate way for people in SPvP to level up without earning any XP directly. All you need was, say, a little gold reward with your glory rewards.
But not all crafting disciplines can be leveled with gold alone. Cooking, for example, require materials that can be gathered around — which you can buy with gold in the Trading Post — and materials that can only be bought with Karma. Now, seriously, would you expect someone to buy those materials with Karma and sell them in the Trading Post?
So, besides gold, SPvPers would require some Karma rewards.
This would keep the Trading Post busy and always with something to sell.
Of course, there is another solution: As you can, today, trade Gems to Gold (and vice-versa), there should be a way to trade Glory — the PvP currency — to Karma. Then, people would be able to buy stuff with Karma and sell them in the Trading Post, earning Gold and using it back to buy materials to craft stuff and then sell those in the Trading Post again, making the Trading Post useful and giving players a “sink” for Glory.
(Or, maybe, you can already buy bags/boxes with crafting materials with Glory and all this is simply an idea that makes absolutely no sense.)