I'm a little surprised at how throwing on ret gear is established wisdom when I don't see any numbers to back that up. From my studies using ret gear really isn't the solution at all.
I spent a lot of time and effort in BC to build a ret-haste set in order to dps while prot when I wasn't needed to tank. It never turned out well. My numbers were horrible. The simple reason for this is that without the actual talents from the ret tree using ret gear doesn't really benefit you much at all. Simply increasing the swing time didn't turn SoB into some monster for damage.
If you look at things now they aren't really any different and in fact the gear difference is even more pronounced. STR is most highest on prot gear, just research it, it isn't hard. And STR is the #1 attribute for doing dps while prot followed by BV and then AP. This is established very well in another thread here.
Given those facts why would you suggest something use ret gear which has no BV at all and usually AP instead of STR. The stat concentration that benefits us the most is actually found on prot gear and not ret gear. If someone is serious about increasing their dps while prot then what they should do is try and build an entirely secondary set of prot gear that stacks only STR gems on it. Concentrate on gear that is highest on BV and STR and enchant and gem it with STR and more BV.
As prot you don't have crusader strike or any of the things that boosts a ret's dps. What you do have is your shield attack and SoB. With an incredible amount of str you can boost both of those attacks to a fairly decent amount of dps. Unless you are hoping for a great crit value from an all ret set that makes up for that loss in str I don't think that saying to use a ret set is an automatic solution.
Also not all guilds are contentious when it comes to tanking. I enjoy sharing the tank spot light with our other tanks and don't mind the occasional OT spot during a boss fight. If your guild is any good at all you don't need to kick tanks so every last spot is a dps.
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