I'm troubleshooting a problem with our firewall at the moment. It's a brand-spankin' new Astaro 220 and so far, it's been great, but it keeps blocking my superintendent's access to his AOL desktop software. While this is hardly a problem (blocking AOL, whether for IM or otherwise, is rarely a bad thing), he relies very heavily on the calendaring features to get his job done.
We're moving to Google Apps this summer and at that time, we'll migrate everything to the Google Calendar application, giving his secretary access to his appointments, allowing us all to schedule resources with him, etc. For now, though, he needs access to AOL.
For a corporation not to use a unified groupware solution would be unthinkable. This is the way business is done. There isn't a question of who is available when in what room or which office. There isn't a question of forwarding your mail to a personal account so that it's more usable. However, many schools take a rag-tag approach to groupware, using whatever tools might be available and killing the possibility of interoperability.
On the other hand, if we give our users a suite of really usable tools and train them in their use, they won't have any need to install "rogue applications." Why use AOL when Google (or any number of other web-based groupware applications) is available, integrated with your peers and coworkers, and provides as much more functionality?
Kill AOL, but make sure that your users have messaging capabilities, rock-solid email, calendaring (both personal and shared), and online storage options. In this way, you can standardize and provide top-notch support as well as choose a properly vetted and tested set of applications.