In my experience, the customer that brings in the least money also makes up for most of the work. It's a larger percentage of their budget. For a Fortune 500, your fees could well be a rounding error. These small guys are also the ones who would go out and post bad reviews, demand the most features, demand the most uptime and the fastest response times.
Free users also have lower expectations. Low-paying users might not be worth the hassle. If you sell candy by the hundreds of kilograms it's better to give free samples than to sell little bags.
The point being how big of a workflow advantage someone would have from a paid account and their business goals based on that.
Adobe suite is a great example - student pricing, normal pricing, teacher pricing etc etc.