WP is already acting as "attack" and "defence" stat for some mind abilities. So it makes sense doing the same for Mind Crush. Also it would nerf those turn 1 Mind Crush nuke exploits by scaling down damage at targets with full WP. And it would give Priest weapons some synergy with Mind Crush.

The damage formula could be Damage = 100 + 10*(AttackerWP-DefenderWP)

Less damage against Pandorans with full WP and a lot more damage against Pandorans with 0 or so WP.