Is a Defragmentation program still beneficial/necessary in Windows 7?

Since Windows 7 defragments the hard drive automatically, according to schedule, is it still necessary or beneficial to have an additional defragmentation program installed, such as Diskeeper? Is Diskeeper’s ability to prevent fragmentation worth the price of the program alone? Or does it’s ability to defragment outperform the Windows 7 defragmenter by a wide enough margin that it’s worth buying?