Should a startup write a business plan?

I guess it depends on how you define business plan.

Business plans are important because they force you to think about things you need to accomplish to have a successful business. Potential market, competitors, goals, cashflow, etc are all very important to starting a company and founders need to all agree on the these things to progress.

However, I don't think a formalised 50-page business plan with research, etc is really all that necessary. It's much more important to have actually thought about all the things that affect the success or failure of your company than to have documented them.

I have my own business and don't have a business plan. I think that it's hard to come up with a realistic plan when you're just starting out; you just don't know much about how things really work in your market. I've learned a lot, mostly about what doesn't work, in my 2+ years of being in business. Maybe I could use that knowledge to write a plan for the next year.