I am a benchmark tester using Ant everyday. Our environment has 7 test machines and 1 console machine. For each test run, the ant script updates the working copy, restarts the database, build the product and the Grinder with proper properties and deploy them to the machines, and starts them and the monitoring softwares. With the 1000-line-buildfile, I can just type in an "ant A B C D E"-like line in the terminal on the console machine. I don't think it's a good idea to do this in a IDE like eclipse or NetBeans or anything else.
Also, in my company's product, ant is widely used. We use ant to build sample data for the benchmark test; we use ant to update Tomcat which is going to be bundled with our product; we also use ant to build and run selenium tests.
What's more, when you want to release the source code, including the IDE project files is not a good idea because there are a lot of IDEs. And what if the user just want to get a fresh build from your latest code (if you don't provide nightly build)? Providing a set of ant (or other build tool) buildfiles is a good idea. With this, you can manage your classpath for the build.