I've found the elemental fire and water forms rather underwhelming, the primary reason to be in them is the extra 100% fort to negate sneak attack and crits which is sorely needed since that less than desirable move to allow npcs to bypass fort passively (which in all cases rogues should get 1% fort bypass per rogue level as well, its only fair to give what you get.)
Allowing us to take all of the elemental forms as an option would make far more sense. After all the -5 to caster levels out of element is a pain in the rear and does not promote using class abilities. Fires of purity is nice when fighting things that aren't immune or have high fire resist. Each elemental form should have that type of weapon buff as a minimum.
Air elemental form: Grants +3,510 to trip and when you trip you get to throw your enemies around endlessly of course. (joking...probably best to have active feather fall + competence to movement speed 15-20%) +10 lightning resist -10 acid resist, caster bonus for air and sonic spells
Earth elemental form: +10 acid resist, -10 lightning resist, caster bonus for acid and non-type damage spells (dehydration and such) -10% move speed +5% hp or perhaps a sturdy bonus to prr or dr.