> If you really believe your employer's stock will double in value, you can take your cash bonus and buy shares of it.

RSUs aren't quite that deal, because they're "invested" before they vest. Even assuming that the employer would be equally willing to give you either $X/yr in cash or $4X in RSUs over the next 4 years, you can definitely come out ahead if the stock keeps going up. It's effectively a form of leverage through time arbitrage; you get to buy $4X worth of stock at today's prices using money you don't have yet.

Consider a simplified scenario where the stock is flat forever, except that it doubles in a single day when you've been there for a year. If you take the $X in cash and immediately buy your employer's stock with all of it, you only double your money on that first year's paycheck. By the end of the 4 years, you would have $5X (the $4X you earned in salary/bonus, plus one extra X from the doubling). On the other hand, if you take the RSUs, that entire grant, including what hasn't vested yet, benefits from the increase. In this scenario, by the end of the 4 years, you have $8X.

Similar happens with any scenario where the price is monotonically-increasing, but with more arithmetic. If you assume that the stock will go up over time, and that you wouldn't want to be trying to time the market, this starts to look like a pretty good bet. Compound that with the fact that most employers will be willing to offer a much higher number in RSUs than in cash - that is, if they would offer $X in cash bonuses, they'll offer >>$4X over 4 years in RSUs, especially as a starting/signing-bonus offer - for reasons having to do with financial accounting, and it's suddenly a very attractive deal.

The main reason to prefer cash over RSUs is for diversification. But if we're talking about a public company, you can still sell the RSUs as soon as they vest in order to diversify (and the standard advice is to do so). You really only stand to lose if your employer's stock goes down between grant and vest - not just worse than market, but actually down - which, on average, is an easy bet to take. Getting RSUs at a private company is a much dicier prospect, of course, because now you're locked into that lack of diversification in a way that really matters; even after the point where you get the paycheck, you don't get to bail out if the ship starts to sink.