That IS true, but there are three main reasons to buy chickens: meat, show, and eggs. Obviously you would want a purebred for show, and most likely meat (I don't have a lot of experience with that) but for eggs, most people just want an even-tempered good layer. A purebred isn't always the best choice for this. Thinking from the buyers perspective, mixes are best for layers. I also know that blue-egg layers are popular, of which purebreds can often be tempermental (take the Araucana for example.)