
If you have a mixed network like I do sometimes you have to compromise. At my job we run Windows, Linux and a sole Mac (Graphics dept.) and lets face it, when you do consulting work and if you design and develop custom applications you have to be able to develop for your clients platform and as much as I hate it, it's a Windows world. Before I used to have 2 workstations, one Windows and one Linux, or I had to dual boot. In the past, virtual machines have been lacking. Either they were too slow or lacking a certain pizazz to get the job done. Enter
VMWare Workstation 4.
It's simple why you'd use VMware rather than an entire new machine:

1) Your way, I have to buy a new machine for every OS I want to use. Try having to buy all those machines (or even some of them) that you suggested, and you're easily at the price of a copy of VMware.
2) VMware is only expensive the first time. Upgrades to the next version are only $99. Ie, if you buy version 1, version 2 costs $99. So does version 3, and then 4. In my case, I was lucky that my company bought me version 1, and have only paid $99 to get the new versions.
3) Vmware saves TIME. If you have to reinstall an OS on a physical computer, it takes a while. If you create a default Virtual Machine in VMware, you can copy the files to a separate directory. Then, when you break your existing VM, just delete it, and copy the other files back. Voila! Instant Virtual machine in less than a minute.
There are other benefits, but those are probably the best.