That way both gear types are basically equal in the open world and equally handicapped in the respective instances. I see no reason that PvP gear should be weaker in the open world against mobs. Gear differences are mainly been for instances anyway.
We've discussed a similar idea (and may yet decide to go with something like it), but our concern with it is that it's still somewhat complicated for players to figure out. As I mentioned earlier, one of the issues we'd like to solve is how difficult it can be to figure out exactly what impact PvP Power is going to have on your effectiveness. "1 PvP Power = 1 Strength vs Players" is easy to get across and understand. "1 PvP Power = 2 Strength in Arenas and Battlegrounds, worthless in dungeons and raids, and 1 Strength in the open world," not so much.
One other thing I should mention based on some other comments in here is that, just like today, PvP Power will be active vs players in any context, not just inside PvP instances. So if you're comparing a PvE and PvP piece of the same item level -- for example, a piece of Conquest gear and a piece of Heroic raid gear -- it should work out something like this:
The PvE item is stronger in PvE instances
The PvP item is stronger in PvP instances
The PvE item is slightly stronger vs creatures in the open world
The PvP item is slightly stronger vs players in the open world
Once again, I need to be clear that this is all early development thinking and anything might change, but our hope is that whatever we end up doing makes things work
Sujet: WOD ET LE BOUT DU TUNNEL 
Mar 14 Jan - 7:51
