That business plans are a bad thing is becoming a meme.

It's great to get some (many in PGs case) likes on twitter and that's about it. A business plan is great to have because it forces you to think through all the important pieces of the business. Many great companies had one, e.g. Apple (as mentioned by someone else in this thread). Amazon has detailed Word documents for anything they dive into.

Of course, planning should not becoming your core activity... There's a limit after which business planning is just an academic exercise.