if you're using the mouse to click, and the keyboard to move, the only downside is you turn slower, if you click a spell that's instant cast, turning is irrelevant, if you click a casted spell, the cast time allows you to go back to your mouse.

The only reason, and the primary reason I'd use the keyboard is because it's a bit more reactive, you can change your mind about a skill and hit a skill across the screen if its bound where you cant swish your mouse like a mad man.

playing melee is also far more conducive to mouse movement where casting is more lax a decision.

Yeah, that's the point. Mouse move > keyboard turning, though I'm impressed they got 2.6k rating with it.

Well it helps that the rogue is playing the least skilled spec in game at the moment, and they have a paladin and a druid.

I played standard mut/prep before fok became popular and we did just as well.

Ultimately whatever works. So long as it is comfortable for you it might do more harm than good trying to keybind spells you're used to clicking. In the same vein golfers who have unconventional swings or runners with weird gaits will often persist using them even if something else is demonstrably better or more efficient.