As someone who bought Sparrow during their sale last week, I feel ripped off. I don't think they owe me free updates forever, but I would have liked to have known that it was a fire sale.
I paid full price for Sparrow and used it for two months. I didn't like it and switched back to Mail.app. But I don't feel ripped off because I got my money's worth from using it.
The app works, so I don't know why you would feel ripped off, fire sale or not. It's an overglorified EMAIL client for heaven's sake. How much more can you expect from an app that does e-mail?
It continues to function as-is (albeit with no updates) regardless of whatever happens to the company, right? And if it really is as useful as people are saying, on email, which most people spend hours of their day on, then I'd definitely still have bought it.
How much do you value your time? For most software professionals, $10 is not even rounding error.
The app works, so I don't know why you would feel ripped off, fire sale or not. It's an overglorified EMAIL client for heaven's sake. How much more can you expect from an app that does e-mail?