Well, a point. A powerful enough 64bit processer is still, to an extent (I'm not getting into this argument (again or here)), going to waste running the 32bit OS. Particularly the way most people use their systems.
If it were easy enough we could say to people, "look, here's Xen, here's a Windows image. Here's a backup of the image for when you root it with spyware and here's what to click to start it each time." I mean, I really wish it were that simple (hell, it really is with some basic scripting skill) and you could, theoretically, fully virtualise 32bit XP (or 64bit, or whatever) so they could use it properly.
The real beauty that I see from the 64bit/dual/quad push that's going on is not the extra instruction set (okay, it's pretty neat but there aren't enough people using it to make much of the code worth it, I do get your point), it's the virtualisation (as above).
My mother, for instance, would be a perfect candidate. Get a decent company to do it, set aside a 2GB scalabe image for Xen with the Windows install on it (Vista Ultimate/Business/Enterprise or XP) and when they root the system they can start again, backing up using My Documents which is mapped to the main disk, rather than inside the Xen instance... Whack on something like ClamAV for some base level security on the Docs-Settings/users folder and you've got yourself a very basic setup that I honestly think people should look into more...
It is, to an extent, what I will probably setup on my laptop (except if it's Linux it'll be a primary Linux install with a virtualised install for dev work to stop those pesky dev utils running on boot and playing the Battery-Vampire role).
IF Quantum computing gave us more power to do things like that, I could see it working greatly in the desktop environment's favour. The other suggestion is ... cloud computing? If we all end up with dumb terminals and massive ISP plans, modem is replaced by a dumb terminal with a build in pseudo-LAN card-thing (okay it's still a modem) then would we claim it's a replacement for desktop computing? If the Quantum PCs take over the "cloud" world, would it count as a replacement?
I'm getting bogged down in semantics now, but I think you get the other argument that could be made... in that sense I guess the answer would be yes, at least for anyone working in a company that might have cloud computing (unis, big companies, govt., etc..).