Here is one simple threat, happened to me using VirtualBox but same applies on VMWare.

Say you got 10 Linux Machines on your startup. Your developers are all doing their development on the linux but have to test the product on windows as well. Moreover, you need to author/edit/view some files on windows/ms office.

Since you are running your windows/ms office instance on a virtual machine, you will need to buy a single license for the product and then once you had it activated, you can simply clone the machines across the network having each one of your developer his/her own copy of this windows instance.

In this case it might or might not be legally permitted but there can be a virtual product which breaks the licensing model without breaking the law.