The reason to defend the definition of "open source" as it currently stands is to prohibit organizations from profiting from the accumulated goodwill of the open source community and movement without actually adhering to the ethical standards of that movement. We've seen in the past that companies will release source under an encumbered license and claim to be "open" as a result -- the FSF and OSI were founded in response to that type of maneuver.

As far as I can tell, it's been a success. The recent batch of nonfree licenses have all had to emphasize that they are not open source in the face of public backlash. Making euphemisms like "source available" unappetizing is exactly the point.

Furthermore, the OSI is just a group, as is the FSF. If the OSI and FSF folded tomorrow the definitions of "open source" and "free software" would not change. It's not necessary for the OSI to approve a license for the software to be open source, so "OSI Approved License" is not sufficient. "Licensed on terms compatible with the Open Source Definition" might be more accurate, but "open source" seems like an adequate shorthand.