This happens all the time. People mention a few software packages, in whatever realm, and don't mention the ones that are the biggest competition--the open source ones. Yes, some open source software has such a low adoption rate that it hardly seems worth mentioning, but in the CMS area, Sakai and Moodle have both cornered a pretty significant market. Sakai is used at big universities like Michigan, Indiana and Berkeley. That's hardly something to sneeze at.