I am so tired of people saying they NEED dual spec. No you do not.
I am an Commando, specced Healer. I love it. Yes it takes me longer to kill and complete dailies or any quest for that matter. Gues what it should take you longer to kill things. Your a healer.
Why people feel that they deserve dual spec is completely ridiculous. Quit saying we need it. Play the game as it was made and OMG enjoy it. If you want to kill things faster then maybe you should have been DPS specced and not a healer.
Individuals don't need dual spec, the community however does. You see, unless a full 25% of the population decides they want to sacrifice leveling speed to fill that absolutely necessarily healing role in groups, then we have a problem. 1 person out of every 4 needs to be a healer or players sit idle spamming general chat for a healer whenever they want to run a flashpoint. It just poor game design from a community standpoint and its simple BASIC math to realize why.
Now it could be argued that full spec healers aren't entirely necessary especially for the lower level flashpoints for sure. But the higher level ones really push things. Plus if the full healing spec isn't required then why would anyone spec healing? As you said it takes longer to level using it.
We have a similar problem with tanks. I understand and empathize with healers who choose their support role to be a pure healer. I see how dual spec would be threatening to your unique role. But it's just poor game design to have to the community be overly-dependent on a role that is in such poor supply.