This deters folks from PvP'ing all out. Instead, most folks are stand off'ish unless they're following closely behind Da Zerg (TM).
Furthermore, those of us who are in their PvP'ing (Yes, us melee classes especially) are paying more in gear repair costs than other long range classes.
This has been a common coding principal in PvP MMO's for years... why is ArenaNet doing it this way?
Oh, and they could still code it such that PvE'ing mobs in WvW would lead to repair costs but not solely from PvP.
STOP THE PVP INDUCED GEAR DAMAGE.