I kind of always get them unless I can't even grab the core abilities without the points. I could never feel or quantify how important they actually are, it might just as well be placebo effect in many situations. How would you rank them in terms of priority?

+Fortitude is the most useful as lots of enemy interrupts and debuffs tend to target it. +Will is less important (most non-humanoid type enemies like wild animals don't tend to target Will too much). +Reflex is not important unless you are worried about friendly fire from spells.

 if I have a spare-ish point I’ll pick up one for the defense that the character already is doing very well in. 

often, if I have a character that picks up weapon and shield style then the reflex one tends to also make sense.

 

for a wizard, bc I rely on grimoire switching a lot, I have tons of skill points, and I’ll try to pick them all up.

 

They're mostly useless unless you play a tank with optimized stats who can stack defenses. But fortitude is the most important of them.

I take them almost never. Fortitude is the best usually, as just said.

In solo it depends by the enemies you fight, i.e.: if they dominate, will, if they paralyze, reflex, if they stun then fortitude.

But I end taking attack abilities 99% of times and skip them.

Nice to have the option, in case you need to mitigate a Subclass penalty, build for Mindweb, or use your character as a decoy for a specific non friendly AoE spell (there are better way to do that but still). 

Also specific case such as Nalzpaca really hating Arcane Dampener, etc... 

Never wasted, rarely important.