The priorities are often missing crucial parts and sometimes just flat out wrong. In the beginning of MoP this was even more the case. They have since then been updated to include just as much as the in-game "core abilities" tab.
The dps ranking are just wrong. Their sim-craft results are very old (and not optimised) and don't take into account stuff like the 100% crit trinket. Add onto that how even optimised simcraft does not mean anything. Noxxic shows it as if it were 100% correct.
When I looked a few months ago at their Dps-rankings they actually had feral do less dps in higher ilvl. This has since then been fixed, but just shows the lack of effort that goes into the guides.
If you want to "learn your role" so ist on your current specs. If you want the achievement points, do it on the alt/spec you consider the easiest. People have to complain about really everything these days, geez.
Isn't the point of Proving Grounds to prove skill regardless of gear?
No rewards at all. You get a title for endless 30 depending on role, but nothing more iirc