A system that ships with UEFI secure boot enabled and only includes Microsoft's signing keys will only securely boot Microsoft operating systems.
By that you mean only Window$ 8.
I think it really will be a major problem for ALL users. You will be completely locked out. You won't even be able to change a graphics card without permission from M$. It is 100% draconian, typical of M$.
Looks like buying any PC with the Window$ 8 logo is out of the question.