Generally, both Spriests/Boomies are more desirable than Ele Shammies, and the situation will only get bleaker for Ele's once they fix Totem of Wrath's Spellpower bonus stacking with Demonic Pact from a Warlock. Spriests and Boomies on the other hand are both very critical classes to have in WotLK's end-game PVE content (especially the hardest two encounters - Halion and LK 25hc). Even outside of those encounters, both those specs remain very important because of their buffs/utility.
As for melee DPS, pretty much any melee spec in the game now can hold its own to a satisfactory degree. Aside from Enhancement Shamans, and hippy specs like Sub Rogue/Arms Warrior, every Melee spec is competitive and has a useful niche. The only exception to that I can think of is Assassination Rogues, and that's because of Warmane bugs. Unholy DK's are somewhat bugged, yeah, but they're also the only spec in the game that brings +30% disease damage (which basically increases the damage of Spriests by 6%+, and the damage of other DK's marginally), and an efficient means of spreading the +13% magic damage debuff to large amounts of targets - hence why they're often favorites in RS 25hc.