Coming from a GamesIndustry interview with Re-Logic VP Whitney Spinks...
"Giving those updates away was a bit risky without a doubt. Some updates produce more new players than others, and it is often hard to tell what will happen with any given update. It more than likely leaves money on the table, as many players would gladly pay, especially for larger updates.
That said, we wouldn't have it any other way. Perhaps it would be different if our fans weren't as supportive of us as they consistently are. We support them by adding to the game that they love to play, and they support us by spreading the word, sometimes buying more than one copy, showcasing more amazing potential in the game with the content that they create, and just by being awesome.
In all honesty, as successful as Terraria has been, it isn't even what we consider to be the most valuable asset that Re-Logic has. That would be our reputation and the unwavering good will that we have built with our amazing fans."