I should add, there *are* plugins that *attempt* to deliver the true functionality that we want, but they do it in a completely sub-optimal way.
You know those 'meta' fields below every post? These plugins make you create a field called 'other_author' and then fill in the name of the author in the 'value' box. This is clearly an opportunity for typos, typing the author's login name, or their last name, or any other value that the plugin won't work with.
And let's say that you get the fields filled in appropriately. If a visitor wants to see what other posts a particular author is responsible for, they will get varying results.
An example: If Wendy and Rob co-author a post, and 'Wendy' is the author that WordPress knows as the main author, while Rob is relegated to 'other_author' status, searching for Rob's other works will return an empty set. Searching for Wendy produces the results you'd expect. These types of inconsistencies aren't acceptable.
Clearly, in order to do this properly, we have to create a many-to-many relationship between posts and authors. The current relationship is a single author-to-many posts relationship, which just doesn't meet our needs.
If someone can tell me how to override the author-popup and substitute a scrollable list of authors with checkboxes, *without* having to hack the core files, I'm all ears. I'm subscribed to the RSS feed for this thread, so post away.
Or, if there are other CMSes out there that support this functionality, please email me at email@example.com and tell me about them.