You lose the reduced taxes of long term capital gains if you wait, for whatever reason, to exercise and sell the same day. You have to hold the shares for 1 year + 1 day from the day you exercise for the subsequent sale to be taxed as long term capital gains. For even the top tax bracket, the difference is 20% in taxes.
That's correct, but remember that stocks go both up and down. Back in the bubble, a lot of people got very burned: They tried to save the diff between short- and long-term cap gains and ended up losing far more because of variations in price.
To add insult to injury, many got nailed by AMT at the point of exercise. By the time they figured this out, the entire grant would not cover the bill to the Feds.
Companies allowing current employees to exercise options 1+ year before a liquidity event would prevent the tax issue. For a company like Pinterest that is virtually guaranteed to have a good liquidity event, the risk of losing your principal is minimal and you are likely being paid enough in salary to afford the early exercise strike price.
If the exercise event and the liquidity event are not in the same year though, you might not have enough cash to pay the taxes. And if the liquidity event is postponed forever (e.g., market crashes like it did in 2001 and 2008), you're stuck with a big tax bill and no way to pay for it.
You lose the reduced taxes of long term capital gains if you wait, for whatever reason, to exercise and sell the same day. You have to hold the shares for 1 year + 1 day from the day you exercise for the subsequent sale to be taxed as long term capital gains. For even the top tax bracket, the difference is 20% in taxes.