The reason you Don't see Blm going Drk sub is same reason you don't see any Blm using Sch sub right now. Absolutely no defensive ability, Blm are there to Nuke and sometimes crowd control, and the sub is extremely situational seeing as Blm already have stun-aspir-drain natively. That is not the same for Sch, Sch can maintain themselves much further down the hate list than Blm and Rdm using several debuffs and cures. Having an additional stun is extremely useful. On anything nasty we always set up a stun Cycle. And on my Rdm I am a side stun not in order as I am usually constantly casting, And always there for the CS stun when needed.
And as I am sure you already know your Dark skill is much higher than Rdm. I swap in Lightning staff for stuns, I did not have to do this prior to resistance nerf. For speed yes Rdm will win for Acc Sch will win. And as the fight progresses with stun frequency being high a Rdm will flat out be resisted. I was defaulted for providing Accurate info but I believe most people just do not have the experience to provide a good account of how CS Stun was nerfed to hell and back. The first three Stuns during CS will generally go unresisted after that its a crapshoot. Now if Drk sub gave absorb Int I doubt this would even be debatable how good this sub was for Sch.
As it is Drk also offers a stun through Weapon Bash, Gives sleep, Bind, Tractor, And a few others not really noteworthy. Depending on fight the loss of Matk may be too great. If needing a good stun cycle being able to step up means alot. The other down side I see though is the whole swapping between arts as Stun is a spell typically used to respond instantly to nastly spell or TP moves.
Every sub is situational that does not mean you should ignore this sub for what it provides.
Show me popotoe salad.