[Groxx]

Design teams that aren't techy enough for VMs, OSes that don't support emulating X (ie, Windows users wanting to test their mobile site on an iOS device), and testing a large number of variations come to mind.

For instance, how long (and how much HD space!) would it take you to set up emulators for all these? http://www.browserstack.com/list-of-browsers-and-platforms (note that these are in-your-browser remote desktop sessions, essentially. some services are static images, like BrowserShots.org, some are not)

For most people, I totally agree, especially with e.g. the IEVMS script. Pretty simple, low cost, and it probably covers all the variety you need, since it's not too hard to install multiple versions of Firefox and Chrome on most OSes. But exceptions exist, and I'd be willing to bet they're relatively large (absolute) numbers.