The different faces of dual-spec

I've been playing my latest level 80 in World of Warcraft, the paladin, doing heroics to gear him up. I'm not quite sure what for, but anyway. I spent his first emblems of triumph not on items for him, but on heirlooms for the worgen warlock I'm planning to create in Cataclysm. So by the time he had enough emblems to buy something for himself, I had already found several nice pieces for my healing set as loot drops in places like ToC5 or the Icecrown normal and heroic dungeons. So the plan now is to spend the emblems I earned as a healer for retribution gear instead, because my retribution set is worse than my healing set.

My most advanced character, the priest (gearscore of 5.1k after that ICC raid, if you must know), never had that problem. I took the occasional loot drop "for off spec" when nobody else wanted it, but I don't really need them. Yes, at the same item level the perfect item for a holy priest is slightly different from an item for a shadow priest. But it isn't as if the healing items were actually useless for dps. And in several cases the healing item I have is iLevel 251 or 264, and if I switch those out for a dps caster item of iLevel 200 to 219, I'm actually worse off. Spell hit is "optimal" for a dps caster, but not if you have to give up too much spellpower to get it. So for my priest I can switch spec without switching gear, and still get quite a good result.

My warrior and my paladin both couldn't possibly switch spec without switching gear. When my warrior switches from protection spec to arms spec, he needs to switch gear, because all the defense, evasion, and the tons of stamina, aren't helpful for melee dps. And the spellpower and int plate of my healing paladin is downright useless in retribution spec.

My mage can keep his gear when switching spec, but then he can never switch to a different role, all his possible builds are ranged dps, and need basically the same stats.

That is just the four classes I have level 80 characters in, but those already show that dual-spec doesn't mean the same thing for every possible character class. Some classes only have one role to choose from, some have two, and some have three. And some classes need to collect several sets of gear if they want to switch specs, while others can live with a single set. I'm not sure Blizzard understands the term "class balance" the same way I do.