I wish I could find it, but the CEO Phil Libin gave a talk about how to price goods. He split pricing models into 3 camps, based on how they deliver value over time:

1. Fixed price goods correspond to items where you realize the value upfront, then over time the good decays. Like food, physical goods like clothing, etc

2. Fixed price subscriptions correspond to services which deliver constant value over time, like a newspaper or utilities

3. Freemium subscriptions start free, then charge more money over time, and correspond to services which deliver increased value over time. Lots of free 2 play games work this way, with people paying hundreds of dollars as they get serious. He also argued that Evernote was in this camp, since as you store more of your information in Evernote, it becomes more valuable to you

So charging a one-time fee for evernote is mismatched with the value delivered. They would get payment as a one-time event but the more you use it, the more it costs them to support you, without you paying them more because you like/need it more.