One word: Games.
One word: Games.
And sometimes it is a crutch that Microsoft doesn't provide. I recall Windows Vista causing some pain for old games - such as requiring the Screen/Resolution window open otherwise Starcraft would run with corrupt colours. Wine isn't perfect, and I expect it never will be. But short of a full virtualised Windows XP, it is one of the best ways to run those old games that aren't getting treatment from the likes of GOG.
WINE can make a lot of really cool stuff happen. I suggest you take another look.
Disclaimer: I contributed a patchset to WINE 4 years ago.
Also, not every developer/publisher has - or is willing to commit - the resources to release their games on Linux/SteamOS. It's difficult enough to release a game for Windows that runs reliably on the multitude of hardware configurations out there. Duplicating that effort to release on another OS is a massive undertaking for some.
Really it's too important to take down very often. I've considered buying a separate box for Windows. I run Windows applications in either WINE or VirtualBox, but with games that don't support WINE, that doesn't work unless you have specific, hard-to-find hardware and an extra GPU. Since games are the only thing I'm missing out on, more and more games are getting Linux native versions, and I don't have much time for games anyway, I haven't actually pulled the trigger on the separate Windows machine yet, but one day I will.
The setup is still not simple, your common dual-booter will not be able to setup it right, yet.
To get a graphics card actually capable of this, you either need to buy the commercial grade versions (Quadro and FirePro) with are much more expensive, or buy a specific card for this purpose and deal with tons of issues[1] like not being able to restart the guest without crashing the host.
[1]: http://wiki.xenproject.org/wiki/Xen_VGA_Passthrough_Tested_A...
But the setup is not a simple one of neither.
NVIDIA attempt to detect the VM and artificially disable the driver, but works perfectly if you tell KVM to hide the fact it's a VM (requires a flag to be set in the config).
the technical difficulty of pci passthrough consists of clicking "add pci device", then selecting your graphics card from the list.
So then you run Linux VMs in a Windows host. In my experience, VMware is a better choice on Windows hosts. But it's been a few years, and maybe VirtualBox is doing better on Windows.
I know a few gamers who would like to relegate Windows to a locked-down VM ;)
But do not take that as discouragement, your friends definitely should try it on their own.
EDIT: Yes meant to answer the "Why dual boot when you can use a VM". My post meant to say "VM is cool is Windows is the host". Sorry.
Sorry about that.