The ISO -> NSO switch can have severe tax consequences for the employee. Even if your company doesn't expire your options in 90 days, consider exercising within 90 days anyway (and talk to a tax lawyer, etc.).

One not-as-obvious reason why companies are reluctant to set long expiration dates on options is because it means former employees take up space in the cap table even if they have no intention of ever exercising that option. The company essentially has to treat those shares as having been purchased, without having received any cash for said purchase. Cap table cruft can make it harder to negotiate subsequent funding rounds.

Oh noes! Don't disrupt the almighty cap table with a few peons worth of shares!

Please. These are discounted to zero by anyone worth a salt.

In this situation it would be great if there were something similar to the 83b letter for non-founder employees. I know too many people who have exercised options when leaving a company (and paid a hefty tax) only to see the value of the resulting equity trickle down to 0.

Yes, but if the 90 day expiration is in the plan, then it is 90 days. It doesn't matter why the lawyers put it in there, though IRS treatment is a reason why it is very common. You aren't going to be able to negotiate changes in the stock option plan with a company you are thinking of joining.

If you are working at a company like Pinterest with a more employee-friendly option plan, then you should value the stock options at a higher value as compared to less employee-friendly plans. I ultra-long expiration periods are awesome, and they are another practical method to deal with the longer period to IPO. These types of options aren't as good as having liquidity for the stock, but with them you can be sure that if you vest the option and the company value goes way up, you'll get the benefit from that.