Any reason why people just aren't using bootcamp??
Bootcamp is great if your Windows instance absolutely needs all the resources of your machine, as in gaming.
However, today's personal computers are so powerful that you can get amazing performance running Windows (or Linux, or what-have-you) at the same time as OS X. There are many advantages to doing so, not least being that you can hot-key between OS X and Windows (or Linux or...). It's like having a desk with two computers on it.
Or more: my rMBP is powerful enough to run several virtual machines at the same time. So I can have Windows XP, Windows 7, any of several flavors of Linux, all accessible simultaneously.
In my work it's necessary that I have reasonably quick and portable access to different versions of various OSes with different versions of various software. Virtual machines make that possible. I have a nice USB 3 external drive with my less-used VMs on it; I just velcro it to the back of my rMBP's display when I need it. Connected via USB 3 or Thunderbolt, the VMs run at native speed. (For external storage, USB 3 and Thunderbolt are pretty evenly matched, as the drive is the bottleneck in that case.) If I'm using a particular VM frequently for a project or something, I'll copy it to my rMBP's SSD. Currently I have four VMs resident on the SSD.
Within the last hour I've had Win7, WinXP and Linux Mint all running at the same time as OS X. Can't do that with Boot Camp.
If I don't need a VM that's currently on my SSD, I can just copy it to my external drive and reclaim the disk space in my laptop. Can't do that with Boot Camp either.
If I screw up a VM, I can just pull an earlier version from my Time Machine backups. Can't do that with Boot Camp-- lots of folks don't realize it until it's too late, but Time Machine doesn't touch the Boot Camp partition. Or, I can make snapshots of my VMs and revert to them if desired. (They take up beaucoup disk space, though.)
I have VirtualBox too. It has improved greatly in recent years. Its USB handling still lags behind Fusion's, but it may be fine for your purposes and is free. If you're not a gamer, I'd recommend it over Boot Camp for running Windows or Linux.