I put together balancebadge.com, a simple bitcoind related app. If I didn't use the chain.com API I would have had to run a server with bitcoind (~20GB of data), a server that indexes data from bitcoind (recall that a simple address balance is not exposed in bitcoind), and stores the data in an alternate database (~200GB). There is no way that I would have built balancebadge if I had to setup and maintain 3 servers with 100s of gigabytes of data.

Also, should developers who don't want to run a PostgreSQL server think twice about building web apps? I believe that general purpose software that requires server maintenance and state management should be run as a service. App developers should be focusing on the details of their product –not how to keep general purpose infrastructure alive and well.

edit Full disclosure: I am one of the creators of chain.com