When to swap from Protection warrior to other dps spec during levelling

Hi

when does the Protection spec deals less damage than otehr dps spec (furry, arms). This bugged spell scale confuse me a little bit.

Thanks for all your answers.:P
It begins to pay of after 80lvl because because mobs pre cataclysm doesnt have so many HP. Protection warriors deals immediately burst dmg which is enough to 2shots mob with 15khp.