You seem pretty intent on drawing a negative picture of people who can do both design and code, labeling them the worst designers you've ever worked with or a jack of all trades whose ultimate route is some idiot manager role.
There's nothing but benefit for a designer to also pick up some coding skills or a developer gaining an appreciation of design and interaction. And I see no reason someone can't excel at both if they truly desire.
Perhaps the worst designers you've worked with did both, but don't let bad logic lead you to think that the best designers do not.
It takes 10,000 hours of practice to become a good programmer, it takes a similar amount of practice to become a good designer. Yes, if the person spends 20,000 hours, they might be able to do both well. If the designer just learns some action script and never practices, then they aren't really learning to program.
I guess you guys live in a hiring environment where good designer/programmers are easy to hire. For us, just getting good designers, or good programmers, is hard enough. The ones who claim to do both are not good enough at either to make it worth our time. We also suffer from a shortage of good designers compared to good programmers, so dev is always blocked on design and even if designers could code, they would never be given time to do so, they just so many other things to do! After a few years, their programming skills would become stale, and it would just be a waste. You sort of have to practice continuously to keep your skills up.
There's nothing but benefit for a designer to also pick up some coding skills or a developer gaining an appreciation of design and interaction. And I see no reason someone can't excel at both if they truly desire.
Perhaps the worst designers you've worked with did both, but don't let bad logic lead you to think that the best designers do not.