The reason you don't want to breed the hybrids isn't so much negative traits, it's unpredictable traits, which to many people is negative. The main difference between breeding purebreds and breeding hybrids is predictability. If you have a well-managed flock of purebreds that you've properly selected for specific traits for generations, chances are the chicks are all going to have the same characteristics as their parents, because you've already eliminated all the variables that you don't want in their genetic line. On the other hand, if you breed hybrids, which are a mix of two purebreds, then anything can happen because the genes will be mixed in an unpredictable fashion. The original two purebred parents will create a predictable hybrid offspring (the Sex Linked Red for example), which is the F1 generation. But when you cross two F1s together you get some degree of unpredictability in their offspring (F2 generation), and when you cross two F2s together it's basically a genetic free-for-all.
Now that doesn't mean that the chicks will be horrible. They'll just be unpredictable. You may get some fabulous birds, or some really undesirable birds. Just like with any mixed breed dog of unknown parentage for several generations, there's the potential for hybrid vigor, or disaster. If you like predictability, you shouldn't breed hybrids (F1s). If you like surprises, then see what happens, as long as you're comfortable culling.