Great software for half off doesn't seem like a scam to me. The app still works.
"I didn't buy Sparrow thinking I was paying for a piece of software that was feature-complete. I bought Sparrow mail as a piece of innovative software along with the promise of lots of great future updates to come."
I agree with this. I bought Sparrow when it had a pretty big bug with folder management on my IMAP account. I emailed them and they fixed it in an update, but there was still lots of room for Sparrow to grow (If Apple allows them to do push, other integration with the OS like Siri, etc).
It looks like Sparrow will turn into the Gmail client for iOS, which is a shit sandwich for everyone who doesn't use Gmail.
Then they should've put it on sale for $4.99 today, after the announcement, with a note in the description that it will no longer receive new features.
If it's indeed a deal, surely all the people who bought last week would've bought this week.
But imagine that you just bought either Sparrow a few days ago, or even worse, made it part of your workflow.
It sucks.
On the other hand, it's always an inherent danger with proprietary software. Open software gets maintained by Debian for like forever ;). I learnt that lesson after the demise of Be Inc.