If you are a duathlete you have to train both disciplines, that's the nature of the game. But at the same time you cannot expect to compete at an elite level in one of those disciplines. That's just fine because you aren't going to be racing against elite cyclists, you'll be racing against other duathletes.

Although I will never run as fast as a triathlete as I did when I only ran, I have way fewer injuries now, I like my body better and I think that my general conditioning is very good. So there are great benefits to cross training. But it's true, if you're a cyclist and you are serious about doing well, running can impede your performance and it does nothing for injury prevention IMHO.

The goal is to be as good as you can at the activities that you want to compete in.