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 think you're right. Planning is worthwhile; the business plan becomes a time sink when people focus on making it long and perfectly formatted, and have financial projections out for many years. The reality is, a startup is still looking for its business model; all the things that flow from that will change if the model changes.
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.
There's definitely value in re-evaluating once you've been in business for a while: some things you have to learn things by experience, so after you've learned them you can plan better.
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.