I'm not sure I totally agree with that. My first example is the ridiculously wide-scale enterprise adoption of Microsoft Outlook and Exchange -- sure, there's a bunch of features that are piled onto those "email clients," but at the heart of it, people are still paying for this stuff.
The second example is Sparrow itself. If people weren't paying for e-mail clients, then would Sparrow have such a huge, dedicated userbase? Or is this just the vocal minority we're hearing from?