Guess, its another of these cases, where the armor system shows, how much it disable the crossclassing, for which the first game was famous. Same goes for chloroform, which is a scoundrel skill that effects magic armor, thought most of scoundrel checks with physical armor. Or Medusas Head which does magic damage and inflicts petrify, but scales with strength and is close range. Or about 80% of all special arrows. So in general those skills are only usefull, when you know, that you will be able to CC. The damage itself is hardly important. I can imagine, a warrior with earth quake would be an even more unstoppable cc machine, than he already is. That earthquake also can apply slow makes it even stronger, I guess.
The problem with hydro & air is the simple fact, that those are pretty low damage class therefore taking down magic armor is a real chore. Hydro-air got as much screwed by the armor system, as by loved aero-rogue got. Also electrified surfaces are less reliable, because they discharge easier and are low on damage anyway. Fire poison/oil still has the huge 'blow everything up' bonus regarding damage.
Not sure about your idea with air and geo, those classes offer no real synergy as far as I know. Contamination kills all the electrifiable surfaces and increases change of friendly fire, not to mention, that all undeads would love poison.
