Firstly, Molten Blast is no longer in the game. Not sure when it got removed, but it's not there.
Secondly, you don't have to spec in air magic versus fire magic, etc. You can have them all. In fact, you probably SHOULD have them all to be the best shaman you can be! Every ability that's listed that isn't a talent should be in your ******** All you need is cash.
For example, I can't imagine having to choose amongst the "shock" series of spells. Flame Shock, Earth Shock, and Ice Shock are all great spells. Flame Shock is easily the most efficient of the three, so long as all the DoT effect is used. Ice Shock does heavy damage and slows the target, sure, but it's a mana hog; I reserve it for mobs that try to run, or if I need to "front load" my damage (i.e. can't wait for Flame Shock's DoT). Earth Shock is the least efficient, but against any ability with a cast time it's GOLDEN! That 2-second interrupt doesn't sound like much, but it saves you a ton of health and trouble. You don't even have to use a high-ranked version of it; you get the same interrupt with the first version as with the final version. I typically went with the version 2 levels below the most current-- keeps the mana cost down.
This is just an example. Truthfully, a shaman has to use a lot of spells and abilities to be at his best. That's the fate of a hybrid class. We've got specific counters for specific situations, so we have to have every option open. Pay the extra money to learn the extra ability. It's worth it, trust me.