Here's what I would consider valid reasons not to virtualize.
1) If your host installation or hard drive is corrupted, or fails, it breaks all of your installations.
2) You are subjecting your environment to the overhead of running two operating systems. Essentially, all the resources you spend on the host are wasted in that you can't use them for the actual tasks you're running in the virtual system. This is much more of a problem the older the machine is -- laptops from as new as a couple years ago may only have 2-4GB of RAM.
3) It's more complicated for the user to set up. I would say that it's much easier to choose an OS from a menu at boot time than have to learn how to manage virtual machines. Navigating hardware passthrough for virtual machines has been somewhat tricky in my experience.
4) Gaming is much more difficult on a virtual machine.
5) It forces one to give the new OS a more full-fledged try, without the temptation of just doing all the work in the host.
And some valid reasons to virtualize.
1) Portability. You can move a virtual OS between hosts and it should work essentially the same.
2) Ease of testing. If you just want to play with something briefly, it's often not worth rebooting. Likewise if you want to test a program, test an OS, virtualization can be useful.
3) Ability to switch between several different systems at will.
Now, not all of these arguments are necessarily valuable for any one person. For an average consumer, I don't think that running many different OSs in VMs is necessarily useful to a consumer. Personally, I would run one host and one VM, and when I'm done with that VM (after days, months, whatever), I will either set it aside or blow it away. I'll then install the next OS I want to test out. Unless one has a powerful system, and can run multiple VMs at a time with decent system resources, the need to switch between VMs becomes essentially like dual booting again, only without a power cycle.
When I built up my OS, I did a minimal install of Debian Jessie, and added only the packages I needed to make the system work the way I wanted it to. The result was a very slim, very lightweight OS that allowed me access to the maximum of hardware resources. Running such a system as a VM on top of Windows would eliminate the benefit of a slim OS by requiring me to run Windows, which is a bit of a resource hog. I have tried using VMs for gaming in the past, and it hasn't worked, so unless I've missed some major revolution in VMs in the past couple of years, it necessitates either a separate system or a dual-boot setup to allow me to game. It would also be nice to have a separate system for the purposes of gaming, to eliminate the need to dual-boot, but a second system unfortunately isn't financially feasible for me at the moment.
I agree that a dual-boot setup isn't ideal for someone who splits their time relatively evenly between two or more environments. You do spend a lot of time rebooting that way. But personally, when the only time I make it into Windows is to play a game, the extra 20-30 seconds of boot time both booting into Windows and back into Linux afterwards is worth not having to deal with the overhead of Windows the other 99% of the time.