I honestly don't know why some people can't grasp the concept of multiple sets.
Having problems soloing single mobs or casters? Have a ret set with you. You won't kill things as fast as ret, but it will be MUCH faster than prot.
Not just a weapon or a few items, I mean an entire set. Leveling as Prot, I kept a ret set and a tanking set with me, and I quested the majority of the time in my ret set, throwing on the tanking set only if I really needed to. Reckoning is nice with a slow weapons, surely not optimal, but with Reckoning +Seal of Blood (or Rightousness if you are ally) I would knock out a good 30-50% of the mob's life depending if I got crits.
Then, if you run into a quest where you have to kill a ton of melee-oriented mobs, throw on your prot set and grind them all out real quick. Gotta kill a specific mob (especially a caster)? Throw on your ret set.