So, I'm a long time monk on FF14 and while I love the class and feel a sense of accomplishment when I play it well since it's possibly one of the harder classes to play at a high level (what with the positionals, optimal use of Perfect Balance, buff management, etc), I've been bothered by the fact that three of its moves don't seem to have very much use compared to other classes (most of which have a frequent use for all of their moves) in PVE content. Those moves are Arm of the Destroyer, One-Ilm Punch, and Riddle of Earth. Unfortunately, the character count was too long for one post, so I'll be separating it between this post and a reply post soon after. ^^;
Riddle of Earth is a 30 second defensive oGCD with a 60 second recast that switches you to Fists of Earth (which reduces damage by 10%) and grants Earth's Reply when hit, which reduces damage by an extra 10% (20% total) and refreshes GL. (On a side note, you'll lose the buff if you switch to Fists of Wind or Fists of Fire. Also, the refresh only happens once).
To be honest, I've found this move to be half useless ever since the beginning of Stormblood. I say "half useless" because the GL refresh is actually really handy during times when the boss is un-targetable (and there's nothing else to hit) for a short time. That said, this is only useful for a handful fights. The recent changes to Perfect Balance and Riddle of Wind make them better choices for saving GL (which you really shouldn't be in danger of losing so long as you can hit something), but it's still useful nonetheless. The thing that makes it half useless is that you only get the damage reduction after the first hit, which means, in terms of saving you from dying to an attack due to not being at max HP or messing up a mechanic in a fight, it's not much better than FoE (meaning you will most likely die). Now, this may be useful for solo content where you're usually getting auto-attacked by an open world enemy, but once you hit level cap (or do a dungeon for that matter), the defensive boost becomes pretty useless. It almost never saves you when you need it most from a raid attack and in the event that both tanks die (which usually result in a wipe anyway) and aggro switches to monk, you'll last 5-10 seconds at best with adequate healing unless the boss uses a tank buster.