But in the case of say an open source library, how likely are clients going to update their code with each new release of the library? Typical case is to do it from time to time, especially with compiled code / shared libs, and expect a few incompatibilities to sort out and fix when you do.

In a fast evolving large team project, seems it depends on how well the boundaries of the subsystems are worked out. Misusing supposed-to-be-private members is one minor possibility amidst many challenges.