Just tried this out and Alpaca is an awesome tool. However, I'd never want to release this in production without tests. Alpaca doesn't generate tests, so you're back to maintaining the tests for your N different platforms/languages. But you'd have the same problem with its competitors, too; Thrift [0], for instance, doesn't generate tests either.
Overall, I'm not sure that the time savings is as big as it first appears, but I think it's great for quick projects.