Perhaps not a Business Plan, but spending a day planning how you'll do business is a good idea and not one that "sucks time and energy".

Let's say [as a concrete example] you're going to open a hairdresser. You need to sit down and work out your fixed start up costs (shopfitting, deposits, licenses), your monthly outgoings (electricity, rent), and what you can expect in income. You need to know the location, the foot-fall, the number of employees, and add in contingencies.

All of that is going to take you a day or so to work out.

This is not a "Business Plan"(TM) with exponentially increasing graphs and 5 year projections, but it's a sound concrete Plan for a Business which lets you know if it's worthwhile starting off.

This is not a high tech Hacker News example, but much of the same fundamentals apply for any business or project.