I basically agree with you, but Ion said in the AMA last week that they do in fact balance like this: niche-oriented specs are balanced to reach the throughput of the other, "complete" specs in their respective niche situations. For example, if spriest can cleave 3-4 targets for the whole fight or if elem can turret into recurring adds spawns, they should reach the DPS of the other specs. If resto shaman can heal a tightly packed raid at low life, they should reach the HPS of other healer specs.
Of course this leads to rogues, hunters and afflocks to absolutely dominate most tiers as niche-less, "complete" specs.