Pay close attention to what Microsoft is actually saying ...
(1)
If you are running Windows-8 on the machine, Microsoft wants to be certain (basically...) that a rootkit can't too-easily be running underneath it. They want their system to be the only OS that
can run on that machine, so that the protections afforded by the operating system can't be "root-kitted."
(2) If you are
not running that system, "it's your computer ... do as you like."
Nevertheless: "Root-kit defense" is a legitimate concern that by-the-way is shared by
every operating system that's used in a commercial environment ... not just MS-Windows. Secure boot, and its various brethren, are technologies that were created in response to customer demand. They are not designed to "lock you into Microsoft," but rather to "lock you in to the system that you
intend to be booting on this hardware, at the exclusion of any others." Such a system
has to have "teeth."
Sometimes that night-time sysop turns out to be an industrial spy, and if "all he needs is a Knoppix DVD in his back-pocket," your entire computer center is basically defenseless.
And
that won't pass muster with, say, HIPAA, or Sarbanes-Oxeley, or ... Therefore, this boot-lock technology is being adapted to Linux deployments, too. Apple's busy with it also. An industry-standard consensus will emerge,
be-cause it is legitimately needed.