Startups involve having to do everything with a tiny group of people, so the more skills you have personal experience with the better. If you business depends upon your product, sales and marketing skills, and you've only been a developer, it's going to be a very tough time.

Working at a startup is a great way to get some experience in several other domains, especially if you are one of the first employees. Even if you don't do it first hand, you'll get to spend a ton of time with "head of sales", "head of marketing", "head of product", because they'll be the other three dudes in your office.