Business plans should be required. Without them, you really can't measure and track the success of things. It's a goal sheet of how you will accomplish things.
As far as tweaking and modifications - that's done from time to time. Normally when we hit a milestone of some sort, we tweak things. It could be income based, client based or time based. We also have things setup as a phased approach, so once everything in Phase 1 is completed, we can move to Phase 2.
Banks certainly love business plans if you ever need to apply for a loan, and if you ever plan to have an investor or partner, you should have things setup there too.