60 Days at a Startup
blog.mixpanel.com
blog.mixpanel.com
1) To learn. If you join a company with bad founders, poor communication or inexperienced members you can still learn, but there are barriers to learning that are unnecessary.
2) To have a significant stake in something new. If your team does not value you, it's easy to get discouraged and stop caring about the product or to feel marginalized and worker-beeish: If you wanted to build something for someone else, why not just work for a more established company?
3) To build work experience. If you don't fit with the team or the team sucks, it's going to be really hard to put on your resume for your next venture/application.
In short, at an early stage startup, your place on the team can literally make or break the product. If you guys work well together, the product will fail or succeed on it's own merits; if you don't work well together it can doom the company.
Be smart.
It's like being a parent, really: Reading every parenting book on the planet, getting a degree in Child Development and attending lamaze classes won't make you a good parent. Being a good parent will make you a good parent.
By all means strive; but by the time you're ready you'll probably find months or years of being a cofounder behind you.