In short: If you don’t like the idea of dual spec, don’t use it. Likewise epic flying mount, badge gear, welfare epics, ans all the other topics that players like to gripe about. I like to think of World of Warcraft as a game that I enjoy, not a system of mechanical restrictions. Anything that makes the game more accessible is good, particularly as this has little negative impact on the players that choose not to use it.
eg. If your raid leader wants a Healer and you’re spec’ed for Priest DPS, then you can’t go. He can ask you to re-spec, but you can refuse. Your choice, and if you keep to a strict “not holy priest” choice then you’re still happy. You have not lost any options. The raid leader will go find another healer anyway. But if you can switch modes, then you get to do the run too.
Why is this bad? See also Blog Azeroth shared topic.