Not junk, just a different purpose. The Arduino is a real-time controller and can respond to external data quickly which makes it ideal for robotics and autopilots. The Pi is a Linux computer. It's primary purpose is to inspire young minds to experiment with software. If you go to the Pi foundation's website, you will see that education is it's primary market. I use a Pi here as a network storage controller. Fairly easy to set up and draws practically no power when not in use since I am using "green" drives that power down after a few minutes of inactivity.