A more thougthtful response on reflection. I actually like the idea, certainly in comparison to the "torture your customers with an ad-infested, miserable user experience, and hope they will pay ransom to make the pain go away" approach. In this case you are offering your subs an enhanced service (post an avatar, or whatever) that they can chose to pay for, or not. In the case of the ads, you are destroying the appeal of your ownn product. That's a terrible model! It leaves the door open for new entrants and will eventually fail.
Speaking for myself, I've grown to detest the user experience (especially mobile), yet I'll never pay to make pain, that site owners chose to inflict, go away. The community/content keeps me coming back, but I think about it now. It's no longer an instinctive, check CIS for the latest...now I deliberately check less frequently, and look for news on other sites. That's not good for your brand.