Not strictly true, the biggest reason to equip PvP gear in instanced PvP content during MoP (which is when PvP Power existed) was the downscaling imposed on PvE gear.
In wpvp PvE gear was superior, PvP power was underpowered in comparison to normal stats.
Nobody gemmed PvP power in favor of STR/AGI/INT, even for their purely PvP intended gear.