As Karl Heinz said, you don't need Reader when you have Acrobat installed.
However, at least if you are on Mac, having Reader and Acrobat running concurrently offers the advantage to have the development environment (Acrobat) and a testing setup (Reader) active. Or, that's what I used to have until I put the development documentation on the iPad, having documentation open in Reader.
So, having both has advantages if you are a serious developer with Acrobat.
Hope this can help.
Max Wyss.