The main benefit...
...of using a VHD is that it is easy.
You simply create a file rather than a repartitioning your hard disk.
When you are done with it, you just delete the file, no repartitioning.
Of course, repartitioning your hard disk in Windows 7 is a lot easier than it used to be, thanks to the new features in Disk Manager. However, it is a bit more time-consuming.
So really, choosing a VHD over a partition boils down to whether or not it is available in your version of Windows 7 and what you feel more comfortable with.
(Booting from a VHD is only available in Windows 7 Ultimate and Enterprise)