In practice, unless the cost is so cheap that doing an 83b is not hard, then an RSU is no different than an option to an employee. The employees will only exercise when there is some sort of liquidity.
No employee is going to have the capital to exercise any other time either, because it's only to get more expensive with AMT as time goes on.
By RSU I mean stock that is granted only when you have met the time requirement as in normal stock options, and when the company is liquid (acquisition or IPO). Cheap stock options that don't expire when you leave also work too.