What (s)he means by this is that you shouldn't treat the OD path as a back up plan. Please, if your desire is to become an MD then just do it so that there's more space for us non-MD, OD aspiring students.
Seriously, if you want to become an MD, then just make it happen. If your true passion lied in optometry, you wouldn't even want to take the MCAT..
I understand where you're coming from--this is a field that you're passionate about, and you feel that everyone who you're competing with should be just as passionate, if not more so. And of course that optometry is in its own right a demanding field and shouldn't be seen as a "second class" career. I agree.
Many (if not the majority of) students in med schools, or aspiring to be medical doctors are unsure about what specialty they want, and there's no stigma surrounding this. Personally, I've known for several years that I want to work with vision and eye health, but I didn't make the concrete decision between shooting for optometry versus ophthalmology until more recently. Before I made this decision, I was contemplating taking both the OAT and the MCAT. Now would you say this means my true passion isn't with optometry?
Having a back up plan shouldn't be seen as an insulting thing. Because of the lower supply of aspiring optometry students, optometry schools ARE less competitive to get into than medical schools, and maybe even dental schools, generally speaking. Whether you like it or not, there are going to be students in optometry school who settled because they couldn't make it in somewhere else. It might be frustrating, or even infuriating... but at the end of the day, you'll be loving your career while they probably won't be.
To
hopeopt: if you're at a point where you're unsure what field you want to study in the future, you should probably take some time out and maybe shadow different professions. If you're not passionate about optometry (which it sounds like you're not, just from your posts), you may end up figuring out your true passion after entirely too much wasted time, effort and money (not to mention taking the place of someone else who truly desires to go into optometry). Would you really want to spend thousands of dollars and potentially four years of your life towards a profession that you won't enjoy?