Why's that? (Not trying to be snarky, generally want to know more)
Downside? You just paid tens of thousands of dollars in hard cash and taxes for lottery tickets. The company could still go under, you'll never sell unless they get acquired or ipo, and you'll be out all that money you spent.
In general, I think it's horrendous advice to forward exercise. Great way to lose money.
Early exercise is a solid choice, provided the strike price is low enough such that exercising is not a major expense / unreasonable qty eggs in one basket.
It's also an interesting quirk of human psychology that we value the loss of something we have (here $10k in cash) so much more than the cost of not having something in the first place.
Seeing smartest people waste their time on this was very depressing for me during 3 years spent there.
If your exercise window is only 90 days, then you need to exercise at least some of your options and file an 83b. Otherwise you'll be stuck at the company until a liquidity event, which can take 10+ years. If the company is successful and you need to leave early for any reason, you're potentially throwing away millions of dollars.