The last 2 startups I was involved in had no formal business plan. Instead they both relied heavily on a solid marketing plan, a rough demo and the team. That's it. Back of the napkin discussions which raised over $1m each during the seed rounds.
As Dwight Eisenhower once said: (Business) plans are useless, but planning is indispensable.
Plans may change or be forgotten but planning is essential.
Business Plans should now be called 'working notes'