I highly doubt SE designs a piece of gear with the idea that it will be Best in Slot. If they really cared about what stats players like best on their gear, they'd stop doing things like putting what people believe is the worst stat to have on the best weapon of patch(case in point being Dreadwyrm Grimoire having Spell Speed). Best in Slot isn't a title that SE assigns; it's a title that players assign. What we have instead are merely options, and having full i130 + the Accuracy cap is always more than enough to curbstomp every piece of content in the game.
It's not in the translated notes we were given, but one of my FC members pre-translated Live Letter XVIII for us way back when and it was directly mentioned there that US/EU largely prefers using Crit/Det while Japan largely prefers using Skill/Spell Speed. Funny how Japan is still pushing content despite the "useless" stat, huh? Either way, both translations I have mention intent to make Skill and Spell speed better regardless of this.
Here's a quote from what we were given for Live Letter XVIII
They want us to have a choice, not be pigeonholed into one set of gear or another by stats that are massively better than others. Instead, we have stats that are barely better than others and a relative inability to render ourselves unable to complete content by choosing the "wrong" gear.With that said though, devotion to raid equipment alone would take away the value of poetics equipment and crafted equipment, so we’ve allocated various stats so that players can choose the equipment they want to use with the needed stats.