Manipulating pixels has its place. Take the classics Worms & Lemmings as good examples. You can also use it to achieve some fancy effects whilst using 2d, such as lighting or colourising sprites on the fly. It can be a better, faster, easier and a less restrictive option than lots of Java2d Graphics method calls.
Of course it's not going to be faster than using hardware, but at the end of the day if you are using 2d you are using 2d.
The first half of jonjava's post makes a great point:
It's potentially a lot faster than java2d - and also in its own way - fun.
I particularly agree with the 'fun
' part. It is fun, and great to play around with. If you are adept at 2d graphics, and not yet a 3d master, you should have a play around with pixel manipulation.