I've used Arc professionally twice. Once as a web interface to an rsync system (http://my-arc-stuff.blogspot.com/2008/02/i-needed-web-app-fo...), the other one as a database for our local library (basic stuff : which book do we own, who borrowed it, ...)
These are real apps. I must admit Arc is really great (even as it is now) for these small, basic but usually time-consuming webapps. It doesn't prevent me for writing a compiler for Arc :) (which is itself written in Arc and is actually the biggest piece of code I've written in this language).