OK, I think there are some terminology issues at play here. I think what you're asking is:
1) Should you create a BootCamp partition and install Windows there, or
2) Install Windows as a pure virtual machine within Parallels.
With Option 1, you go into Disk Utility and create a BootCamp partition on your Hard Drive. Once created, this disk space can only be used for Windows. You then use the Boot Camp assistant to install Windows into this new partition.
There really is only one advantage to creating a BootCamp partition. It gives you two different ways to run windows. You can run the BootCamp partition within Parallels (ie Mac and Windows running side-by-side), or at boot you can hold down the option key and decide to boot your machine directly into Windows. The main reason people might want to boot directly into Windows would be if you wanted to run PC-based games. My guess is that if you're not a gamer, you probably don't want to bother with BootCamp. There are some downsides, among them: it's a lot more work to back up your BootCamp partition. You'll need to use WinClone or something like it to make periodic snapshots, and you'll need to make sure the BootCamp partition is NTFS formatted.
If you're not a gamer, you're probably better off going with the simpler option (2).
Hope this helps.