The "so what" is that Corsair does not have a high item level main hand weapon compared to even red mage and bard now, who have multiple options.
It really doesn't matter what the item level is. Any item level mainhand = increase in m.acc. Whether it's item level 113 or 119, it's still a significant buff, and a R15 plasm dagger/sword is still a "quality" main weapon.
I'm sure you're right that the thinking behind this change was more with mage jobs in mind, but COR getting some benefit is a nice side effect.
This wouldn't really be an issue but Chatoyant staff is still used for cures. many people still require their COR to come /whm. will lead to lost tp and junk. yeah you can not use the staff anymore for anything but im sure some ppl will complain.
Either way its a buff like you said, and its not like ive had problems with resists before anyway
People aren't generally using COR/WHM for their cure potency, they're using it for support on -na/erase and haste. A small loss of cure potency is not a big deal, but less resists on dark shot dispel, occasional sleeps with light shot, and better damage by reducing partial resists on damage shots is likely way more useful.
If you weren't getting ANY partial resists on QD shots before today, I am guessing you aren't fighting high level stuff like Delve NMs/WKRs. Even with a pretty strong M.Acc set it's not uncommon for me to see at least a partial resist fairly regularly in Delve runs. And even if you had no resist issues before, this will allow you to swap out M.Acc/AGI pieces for MAB for damage.
Besides, you should already be using COR/DNC in Ceizak Delve and possibly Foret.