I think this assumes writing an article only benefits the author, when in fact I think it benefits readers far more (if the blog isn't monetized, the author only really gets satisfaction out of it). If I had expertise in some area I would be happy to contribute to someone else's blog, provided I were properly credited (which the author stated he would do).

My blog is still pretty new, so I haven't had to think about this situation yet - I'm the only one who's committed so far. If someone were to contribute I'd be more than happy to figure out a fair way to attribute them, though!