Continuing on from the previous discussion of Proof of Competence, asking for spec is not the best question. It's a decent question. It's easy to answer and verify. And it does tend to give the information you're looking for. But there are better questions.
In particular, asking for rotation is a much better question. Someone who can rattle off the correct rotation is very likely to be a decent player. (Rotation here also means ability priority list, not just strict rotation.) The problem is that asking for the rotation is a harder question to answer and verify. You can't just inspect the player and see their rotation.
But what if you could?
In WoW, there are a number of mods which will display the next ability you should use. Some also show the next few abilities of the optimum sequence. You still have to hit the right buttons, but these mods basically show you the best theorycrafted rotation as it happens.
What if this type of UI element, this "Rotation Helper", was part of the base game?
The idea here is that the game doesn't fill out the rotation on its own. Instead, you would have a screen where you could drag abilities into a priority list. Essentially construct your own rotation. The rotation you constructed would then be displayed, and you could hit your buttons to match.
Another player could then inspect you, and just like they see your gear today, they could see the rotation loaded in your Rotation Helper. And perhaps you could send and receive rotations from other players or third parties. If Sally is another Retribution paladin in your raid and is doing better than you, perhaps you can ask her for her rotation and compare it to yours, or replace yours with hers.
The hard part, though, is to construct a Rotation Helper which is simple enough to use, but also powerful enough to construct a decent priority list. A lot of abilities require things like "use when this buff reaches 4 stacks" or "use when this debuff is about to drop off". This kind of stuff is easy to do in code, in a programming language, but is harder to create in a GUI. A Rotation Helper is a non-trivial design problem.
This Rotation Helper wouldn't really help with reactive abilities like healing, or tank cooldowns, or interrupts. But it would significantly help with DPS.
Of course, there are concerns that this type of UI element is "playing the game for you". But such helper mods already exist, and a decent amount of high end players use them already. These type of mods help those players perform better, and would probably significantly help weaker players who don't know about them.