The fact that some employees have to pay taxes based on the valuations that VCs dream up terrifies me.
For most employees who leave their option grants as options, there is nothing to worry about.
When you are given a grant of stock options, you can sometimes ask the company to let you exercise it early, and vest the shares instead of the options. If you do this when the fair market value (FMV) of the underlying stock is the same as when the options are granted, you will not be in a precarious tax situation. However, many people wait a couple years before deciding to exercise their options, and as a result, they need to recognize a paper gain when they exercise later as the FMV is substantially higher. That's what happened here. They exercised later, thinking the stock price would go even higher, and they were wrong, but they had to pay taxes on that higher price.
While it's true they paid a lot of extra taxes, since they never recognized the gains, they can roll that tax credit forward to cover future gains they may get at some other point in the future. I'm not sure how long you can roll these losses forward, but I think it's for a substantial period of time.