The app server comes with Discoverer, Forms & Reports server, Web Cache and a few other goodies. Plus it obviously allows you to run the app server on a seperate machine rather than doing it all on the database server.
If you have a need for Discoverer, then it's definately worth getting. We use it for this purpose and it works very well.
If you just want to create custom web apps against the database using Java/HTML, then you probably don't need it.