How do you figure? If they've vested they'll make cash.
I imagine this is pretty rare but it's possible for a company to acquire another and strictly buy all preferred stock instead of common stock and then all employees get nothing [0].
[0] This happened to a friend of mine where all execs had preferred stock and were totally fine selling the company and screwing over the employees. Make sure your founders/execs have common stock so that everyone is on the same playing field...
When I left my last company (which acquired the startup I was an early employee at), I did exercise most of my options (all the ones at the lowest strike price.) Just in case. It cost me about $5K. Maybe it will amount to something...