re:
> went back to the original model (joined surface with naked edges)) and tried a boolean
> union with a solid (IMHO nonsense) and guess what happened?
> I have got a a perfect solid object (despite the naked edges)....

You can sometimes get lucky with non manifold geometry if you got these pieces connecting together:



And not like this:



Also if if for containment analysis if it doesn't happen to fire a test ray very nearby the non manifold area it may not trigger any problem at that moment.

But this type of self-intersecting shapes can behave unpredictably.

- Michael