Like most things in economics, it's a risk/reward tradeoff. Taking everything in cash means you're way farther ahead if every startup you work for fails. If a couple succeed (= profitability or small acquihire), it's probably a wash. If even one IPOs, you would've done way better with stock.
Personally I've made way more from stock options than I have from wages. Google stock has appreciated 7x since I joined in 2009, so you can do out the math for various fractions of equity compensation. And this was for a company where I valued that equity at zero, because it already had 20,000 employees, it was in the middle of the financial crisis, its stock was dropping like a rock, and 89% of employees were underwater in their stock options.
You could argue that if you take cash compensation, you could then invest that in public market stocks (like GOOG etc.), and there's a pretty good argument for diversification and not holding your net worth in your paycheck. OTOH, when it's your company stock, you also have an information advantage: for example, the press was very down on Google for most of 2009, thinking that they'd tapped out on growth and the web was basically over, and after the initial post-crisis bump it basically traded sideways until mid-2012. Having seen the energy there, the products under development, and the query growth rate numbers, I knew this wasn't true, but if you'd worked for cash in another company and then tried to invest in the public markets you would've had a very distorted picture of reality.