Top sorc gear is exactly the same for DPS and Healing sorcs, they both come with Willpower, Power, Critical, Alacrity, etc, which benefits both heals and DPS. Any argument against dual specs for sorcs based on gear is a moot point.
I've both healed and DPSed extensively on my sorc. If we were talking Bounty Hunter here, I'd agree that it takes a little more effort to truly get the hang of it, but it's ridiculously easy to heal or DPS on sorcs with just a couple of firing braincells. Healing rotations and a DPS rotations are similar in that they both should try to maximize efficiency (i.e. using Force Lightning and Innervate to maximize D/HPM) while taking advantage of the synergies each tree offers (i.e. Force Bending and Wrath procs) when available. For someone who's familiar with the core mechanics of a class, the only things that change are the abilities you'll use with each spec.
It's actually harder to go from a DPS sorc to another DPS class than to switch roles within sorc. I do agree with one thing though, you'd miss out on some solid damage if you ask a good DPS to switch to healer.
I'm not saying people should change roles, I'm against forcing anyone to do anything. People should play whatever they enjoy playing the most, and if you don't want to play as a particular role, simply don't. My posts were mostly aimed to those who have this idea that sorc healing is extremely complicated and hard to master. It isn't.