Being CC and often in the CP when 1 Pilot is out for a leak or whatever, I chat with the other 1... At the same time, if need be, I could beat the crap out of the 1 at the controls if he goes on suicide mission ...may not come to much but hey, I would not go down without a fight!
Secondly, if the one at the controls has a heart attack ( which BTW, can be sudden) or any sudden critical issue, I'd be the one to open the CP door to let the other Pilot in immediately... The CC may not know to fly an A/C but they are there for a reason.
Leaving 1 Pilot in the CP may not be dangerous, but it certainly is better to have another person to be on guard... Is that bad?
Why feel threatened by them? With the locked door policy, COckpit and CC rarely can have a natter...enjoy it...it would make a good flight...