I think it comes down to what your motivations are--money or fun. Then there are companies like Twitter that have no obvious revenue model when they get started

I think it comes down to what your motivations are--money or fun. Then there are companies like Twitter that have no obvious revenue model when they get started, so do you call it a business? And how often do the Twitters of this world really scale up enough to actually call themselves a business. Given you're a developer, you're going to focus more on the creation of the product. Whereas someone without those skills is going to approach it from a business perspective in all likelihood. If it's just a weekend project, I would say go have some fun with it. But if you're going to invest some time in it as a way to make a living, it's worth doing that ground work before you even get started to make sure people are willing to pay for it.