This is why the WoW community is deemed a foul one nowadays.
People play the game for fun and having have to read walls of texts which you had to dug up for proper/gear/spec/keybinds is a big dent on the experience of the casual player. Why?
Due to how game works right now questing is only really viable when you are a dps. Hence, a class with other specs than dps may end-up in level-cap with only limited knowledge of his/her class outside the spec they played with all along.
However, suppose you are a Priest who wants to try out the healing aspect of his/her class, your first time doing in a dungeon leads into the following:
a) Gets you kicked out of the instance as well as being insulted
b) Making a group of tolerant people miserable
c) Ending up with a poor soul who is kind enough to switch to healing so that (s)he do your role instead while you dps as you used to.
In all three cases you end-up with an undesirable gaming model due to at least a single player experiencing bitterness instead of having fun.
For this reason proving grounds is essential to gameplay as well as being a viable pre-condition to using "Looking-for" tools to enchance the overall player experience.
To elaborate more on this point:
The completion of proving grounds' can be made into the pre-condition for using LFBG/LFD/LFR, to queue for Flex raiding and HC Scenarios.
In turn to make them appealing at the same time, the initial completion of these scenarios can reward half of the equivalent currency for the most expensive main-armor piece.
The examples are as follows:
A priest dings level cap and is required to complete the bronze-level of DPS-proving grounds to queue as dps in LFD and bronze-level of Healer-proving grounds to queue as a healer in LFD. LFD, LFBG and maybe the Scenarios would require the completion of Bronze-level.
Upon completing the bronze for a spec the first time, the player would be prompted to claim their reward of 1.1k JP or 1.1k HP (Granted a chest-piece costs 2.2k JP or HP for PvE or PvP respectively).
For pure dps classes or those that has less than 3 specs, this could be balanced so that entry into dungeons with different specs for the first would grant them 1.1k JP or 1.1k HP.
Druids could get some exclusivity on this or the earning of these rewards would be limited to a maximum of three times.
Same would be repeated for Silver-level which would be the requirement for queuing for LFR/Flex/HC Scenarios.
If you complete Gold or hold a rank in the Endless trial you'd be recognized for someone being capable of normal raids or heroic raids respectively.