Depends on what your goals are as an API designer. If you want a fast and scalable approach, your suggestion would indeed be most appropriate. On the other hand, most users are likely best served with a parser that just returns a single object, which they then can interact with as they desire.
Could that not be easily done as an add on that utilizes the callback approach? The parser library could offer both APIs and the caller could decide which to use and what to do when running out of memory etc.
