I have wwritten a single business plan in my life. And I personally didn't find it very useful (apart from the fact that it helped in finding investment money). I felt very uncomfortable making estimates about game sales based on what I felt to be not much solid data. But I guess that's how it goes in business?
What I did find useful was to write out a long-term marketing plan. And to make simple calculations like comparing the 1% average conversion rate with the number of visitors of our webiste- and being shocked by how little copies we would sell...
Actually that estimate technique might be a bad idea because not everyone who visits the site even downloads the game, typically the rule of thumb is that only about 30%-40% of new visitors download a game, most just there to look at the screenshots to satisfy their curiosity and then go away.
But on the plus side, the more games you release the more your traffic could go up due to more reviews and so on linking to you; your traffic now could double or more after you release a second game. But then again maybe not; my site's traffic (except for occasional increases during new releases) has been stable at about 150-200 visits per day for three years.
As for the original question, I too have created a single business plan and didn't find it too useful.
I think it's a good idea to have a flexible plan and a general direction for your business in mind, and to make lists of ways you want to market and things you need to do next business-wise, but a business plan is not a good vessel to hold that for me.