When you're questing, you are DPS - can't heal mobs to death. Even in the story scenarios where you have to help NPCs kill enemies, it's better to actively DPS their targets instead of healbotting them. BTW, when healing dungeons, I don't have many options either (small heal, big heal fueled by RNG proc, pbaoe heal, instant HOT), but that's a topic for a different discussion.
Which spells are worth to cross-class? Ruin is inferior to Stone II in every way, and I assume so is Thunder except if the DOT can tick fully. Speaking of which, Aero is only useful if I need to cast a nuke on the move, something to put on enemies in dungeons, and for tagging mobs in FATEs.
I'm not convinced it's even worthwhile to cast Aero against a typical quest mob that dies before the DOT can finish ticking -- I could just cast an extra Stone II in that time and do more damage. Or if a mob is casting an AOE cone at me, I'm better off using Fluid Aura to knock it back and spamming some more Stone II. Etc etc.
