Not long ago, a university hired me to evaluate their CMS implementation. They were having doubts about their CMS selection because the implemented system was not living up to the lofty promises that got them the budget for the project. It turned out that they did make a reasonably good platform choice but they failed in two of the other critical content management success factors: expectation setting and project execution. The project execution wasn't horrible for a first try (they were working with a systems integrator that was new to the platform) and there were some remedial efforts already in progress to clean up the implementation. What was really killing them was that a primary goal for the CMS was the notion of "self service." They wanted their faculty to be able to edit their own profile pages. These faculty pages were more than a simple staff directory. Each entry was intended to showcase the experience and accomplishments of the professor and included a CV, a career narrative, and a history of publications. To maximize content reuse, the content model was highly structured — thereby increasing the complexity of the content entry forms. They might have been able to scale back on the detail but the amount of content they were looking for was not trivial: especially for an individual whose chief concern was not keeping a website up to date.
Often, one of the big justifications for a CMS is removing the webmaster bottleneck and delegating content entry to the people who have the information. The implicit assumption is that everyone wants to directly maintain their portion of the website but technology is standing in the way. But if you visit a CMS customer a while after implementation you are likely to find that the responsibility of adding content is still concentrated in a relatively small proportion of the employee population. Either the CMS never gets rolled out to the anticipated user base or it is rolled out and the user base fails to hold up their end of the bargain so the core content managers take back ownership. There are plenty of exceptions to this generalization — especially when there is an individual who has taken responsibility for outward (or inward) communication for his/her sub-group. For example, you might have someone from the human resources department whose job depends on communicating corporate policies to the staff.
You could interpret failed adoption as evidence of poor usability. That was my client's inclination. But you need to consider the usability of the other alternative, telling someone else to do add your content for you. Prior to the CMS, a professor would only have to send a terse, typo-riddled email containing some research references or a CV and the marketing department would clean it up, fill in the gaps, put it in the right style, and handle all of the coordination. How can you beat that? You can't.
Usability can't take the place of ownership and responsibility when managing content. As long as the marketing department owns the website, they will have the motivation to maintain it to the standards that they have set. It would be unreasonable to expect a professor to put the website above his other responsibilities: teaching and publishing academic books and articles. When the marketing department needs to spend a lot of energy cajoling and coordinating a professor for content, it is usually easier for that marketing department to do the content entry too.
Generally speaking, you can't delegate content authorship beyond the perception of site ownership. A good strategy is to centralize content authorship and give a relatively long turn around time (perhaps 5 business days) for site update requests. Those who do not feel ownership of the site will be fine with that lag. But those who are annoyed by the lag (or their staff) are good candidates for becoming content contributors. To them, using any CMS will be preferable to waiting a week. If they do complain, their criticism will be about needing more control to produce more elaborate content rather than needing simplicity for basic edits.
Once you scale back your expectation of distributed authorship, the role of an occasional user gets less important. The occasional user is certainly less important than the primary content contributor role or the engaged users that prioritize website management in their list of responsibilities. You shouldn't buy CMS for the occasional user. You should buy a CMS to maximize the effectiveness of your core users who are primarily responsible for the content and performance of the website.