Quote:

If you rock the boat side to side using a double paddle, you'll do several things: - destabilize the boat and increase your risk of a capsize; - increase the amount of drag in the water due to turbulence; and - induce a slight turning action each time you lean the other way

I believe that but the type of boat has a bearing too.
I paddle a tandem solo and use the double paddle almost exclusively. Fully loaded I get little or no rock becasue the hull is much wider than a true solo and I've learned to minimize the rocking action and developed a paddling stroke that suits my needs and I can do for long periods of time.

I can't speak for everyone, but I know for me that I can go farther,faster and in worse conditions with my double than I ever could with a single paddle.

It's not for everyone but nothing is. I always tell people to buy a cheap double and see if you can adapt to it or even want to before spending big bucks on a top of the line lightweight model.