KMS is not only flicker-free boot and boot speedup. Around a year ago, some people tried to run X-server not as root, but as regular user and it's requires KMS, so KMS can provide enchanted security.
OS: Gentoo Linux, ~amd64. VideoCard: GeForce GTX 470. ASUS P8P67 Deluxe, 2x4GB DDR3-1600 CL9, Core i7 2600k.