Intel VT-x is an instruction set that facilitates virtualization. From Wikipedia
The CPU flag for VT-x capability is "vmx" [...] "vmx" stands for Virtual Machine Extensions, which adds ten new instructions: VMPTRLD, VMPTRST, VMCLEAR, VMREAD, VMWRITE, VMCALL, VMLAUNCH, VMRESUME, VMXOFF, and VMXON.
With some motherboards, users must enable Intel's VT-x feature in the BIOS setup before applications can make use of it
With some motherboards, VT-x technology is disabled by default and you have to go into the BIOS settings to enable it, while with other instruction sets, like SSE, that is not the case.
What would be the disadvantages of enabling this instruction set by default?