1) 90% of our visual front-end bugs come from IE, sometimes only showing up in certain versions of IE7.
2) Our mac developers either need a license for VMware fusion or multiple boxes/VMs to test IE 7/8/9 which of course behave very differently. Maintaining these boxes, Windows licenses, and VMware licenses adds additional stress to IT, and the context switching for our developers is expensive.
3) For the ones that do run a VM, we've had to upgrade their RAM so they could run the app and the VM at the same time. RAM is cheap, but its still money for each new mac-based frontend developer we have.
4) A lot of our distributed application message passing is actually done through the browser via client-side javascript. This makes it even more of a pain to test since automating this is difficult and expensive (more setup code, and capybara tests), and needs to be run in each supported browser.