While there are a couple instances where there might actually be some argument (eg STR vs VIT as a MRD or WAR; there may be cases for PIE as a WHM, etc), the reality is that 80% of the playerbase - and correctly - just sinks all 30 into the single stat. The reason the next 18% are likely not doing that is because they don't actually know that's pretty much straight up optimal for their class.

(And this is, of course, ignoring SCH/SMN who have to deal with the fact it's actually kind of a hinderance to them)

Functionally speaking, it's just a player trap. Unlike Crossclasses where sometimes there are genuine choices and room for player expression, the attribute assignment feels pointless.