Root-kits are the "official" reason. MS (assuming they know about security) knew very well secure boot would be by-passed via malware. It's just an extra layer of false security - but anyway, I'm not that bothered about it... it hasn't affected me yet, since I still use Windows.
You would actually wish to discourage people from GNU/Linux to prop up your sense of elitism!
Why burden the limited support communities with people who are unable to make (or google) a simple "BIOS" change? That's my point, keyboard warrior.