The reason why PVP gives XP and any such rewards useful (to a degree) in PVE
It's to encourage people who normally aren't into PVP for its own sake to pvp. reduce queue times.
I know myself and most of my friends would never touch pvp if they couldn't use it to help them improve their character in some way for PVE.
Sure some people would pvp for it's own sake, but by removing pve rewards from pvp you're drastically reducing the number of people who care, and SE wants more people to do more of their content.