Parsley is one of those herbs many think of as a garnish on a dinner plate. As with other herbs, I am on a learning curve on growing and using it. I do grow more curly than flat for the market. It's never been a huge seller, but perhaps more people using it, or mine is looking pretty with with cooler and wet weather. Some are getting it for juicing as it has good health benefits.
People cooking are asking for flat. (juicers aren't as picky). When trying to figure this out, hear that the cooking shows use flat. Yesterday at the market one of the local chefs stopped by, and I asked, of course. His comment was that curly looks too much like that garnish at Dennys, and he has been led to believe flat has better flavor. FWIW, he uses about 3 lbs/week at his restaurant!
On a note about growing, I think some of the starts of flat I got this spring were already 1 yr old, as they are going into the flower stalk. Not good!