I think the idea there is that one body shouldn't have a proprietary lock on what counts as Open Source and what doesn't. OSI/OSD as the loudest voices of what is OSS, is helpful, but they shouldn't be the only voices.
(As much as I get annoyed by things claiming to be open source, that really aren't proper open source, I have to accept that this is a movement brought about by a rag-tag bunch of hackers who don't much care for single entities telling them what to do, so they should also be measured by that)