I can only speak for my company, but the advantages of equipment diversity outweigh the costs. We've already written "new developer" documentation for both Mac and Ubuntu, which we'd have to do anyway at a certain size. If the site looks bad on any type of monitor, we're able to spot the issue immediately. We can do native testing of IE without depending on VMs. And - last but certainly not least - we're all comfortable with our tools, regardless of previous experience.