While I agree that it is a secondary source, it seems to be a valid secondary source at that.
And I don’t think it’s a crime, I just think it’s rather ridiculous. Why should you have to cut something down that isn’t an absurd length already (550-600 words) just to meet a limit imposed to stop rambling essays that lull on for pages and pages? I just think it goes against self-expression. Common App has imposed their limit on the extracurricular short answer; if they really, REALLY wanted a limit on the main essay, they would do the same.
I also doubt that highly regarded, creative, intellectually-based universities would cringe at the sight of an applicant marginally skimming over the limit by less than 100 words.
But again, it’s my opinion, no more. I don’t want you to think I have some personal vendetta against you for disagreeing with me, and I also don’t want you to think that I’m in fear of my own essay (it’s under 500 words). I’m just expressing my thoughts on the subject. People can feel free to take what I’ve said and run with it or ignore it completely.