>  When I see such a state-checking method that I'm supposed to invoke first, all kinds of questions arise, like "will the answer stay consistent?"

Really, that's what you ask yourself in this case?

> I think any experienced programmer will have some degree of paranoia about the behaviour of the system he's dealing with ...

Now you are doing exactly the same thing you have been accusing me off. Or is this an "technical" argument in your opinion?

> To make a far-fetched example, but possibly real, let's say we want to support OSGi, and that a BridgeProvider implementation is actually a composite of multiple other ones.

Now you are bringing things into play which are not even well defined. What is a "composite" bridge. How would it work and what has OSGi to do with this?

> We don't have plans to do this in practice, but still such things exists 

what!?