Five hundred words is a joke. By the time I write an introduction and a conclusion, I have maybe 300 words left for an actual argument. That's three paragraphs. My history professor assigned a 500-word essay on the causes of World War I. I couldn't even list them in 500 words. Short essays don't teach you how to think. They teach you how to cut. I spend more time trying to cut words than I do actually writing. Give me 1,500 words. Let me actually say something meaningful for once. 
Short assignments are about grading efficiency as much as learning. Annoying but true. That said, learning to make a tight argument in limited space is a useful skill. Op-eds are 500-800 words. Grant proposals have strict limits. This is practice for those.
