2. If the company gets acquired there are several mechanisms founders and investors use to wipe out options holders. It is harder to wipe out common stock owners. If an acquisition event takes you by surprise you may not have time to exercise.
3. You may get laid off, decide to quit, or get fired, and have only a limited time (3 months, often) to exercise your options. That might be a bad time for you to spend the case, so you may want to do it now while you are in a better position to do so.