if you look at the stats closely, yes, shamans are consistently bad on most of the fights. that's what people are trying to get across when they say that "shaman DPS is way too low". (and Balance, before one of the last content patches.) they aren't trying to say, "shaman DPS is the worst, period"
keep in mind that Elemental is the only caster with no spirit conversion talent, on top of bad scaling.
it's not a tax, it's useless stats, bad gear, abysmal scaling and punishing mobility.
i would agree that getting BM, Frost and Subtlety in the mix is a bigger priority than Elemental, at this point, seeing as how a pure can't just switch roles, they're basically shunned away from an entire tree (especially if they don't PVP), which is not good game design.