Cabin crew are different, and more than often during the few occasions we have to socialize with them, we all have been puzzled by their abysmal lack of maturity, education, logical process and general knowledge. The fact that quite a few pilots are married to some is unfortunately shifting the judgment of many.
Basically they are recruited out of nowhere, stay in our companies sometimes as short as 6 months, and after resigning go back to nowhere...
You may encounter some with a little bit of extreme religious involvment - you see what I mean, and for sure I've met some of them which I found a little bit limit for that matter
To give them a function in the cockpit is non-sense.
When my colleague is out to the toilets or stretching his legs - and this last thing in a transoceanic flight is absolutely necessary, and we will continue to do it (in other words no CC is going to clock how much time we spent out of the cockpit, or how many times we go out for a pee) I will remain in charge of who is going to be admitted to the cockpit, and the total stranger who is going to be seated on the jumpseat will not have a word in my decision to open or not the door to one of his buddies...
A suggested SOP : when other pilot is out, and CC observer in, the door will be opened (by remaining pilot) only to the other pilot coming back..... and brief your crew about it, that will deflate some of their self-perceived role.
In other words, cabin crew are much more a statistical security risk than pilots...