Generally the lawyers say the need to divulge keys in hardware is the biggest problem. That's a clear problem for a hardware company trying to secure things.
I guess that depends what you mean by "secure things." If you mean tivoize then no kidding. But what kind of actual security requires the device owner to be unconditionally locked out of the device? It seems to be working as intended.
Backplanes generally that communicate state would be one example. It is mostly things that users shouldn't care about and can easily break things if they try poking about.
I still don't understand why it would be necessary to unconditionally lock the users out. Nobody says you have to encourage them to do it, but why get in their way? If people want to shoot themselves in the foot, it's their foot.
