Small slow-growing companies almost always sell based on financials, and typically for lower multiples than an equivalent large public company would (because their revenue is more concentrated in a few customers and so they're more exposed to revenue risk). I've heard 2.5x revenues or 15x profits as numbers commonly batted around; above this and there's virtually no reason to buy the company vs. invest in public stocks or apartment buildings. On a typical 10-person company that makes a profit of about $300K on revenues of about $2M, this comes out to a sale price of roughly $5M by both valuation measures. Own 1% equity (this is generous, and would usually indicate employee #1 or 2) of a $5M acquisition and you get $50K, which is probably a lot less than the salary you forego over a few years by not joining a big company.
(We're hiring, btw. Email's in my profile.)
That's diversification, all the upside and none of the trouble of actually doing any work.
I agree with most of your points except this one as it's very situational. Maybe not as an intern, but having real albeit small equity in one of these tiny things can be negotiated, and they can get sold. Unlikely, sure, but the possibility is there.
But in general it's about what you're optimizing for. You're points are right for optimizing for intra-company career advancement, resume material, or party cred. But if you have other pursuits, especially with side projects, kids, or just general comfort it can be a good choice.