I found that if I turn on FileVault my keyboard backlight doesn't come on at initial Login screen from boot. Nor can I adjust the brightness of my screen or keyboard backlit. The moment I turn off FileVault I can then adjust those things and everything works fine.
I say initial because if I log out of my user and am taken to the login screen I can adjust both keyboard and screen backlights.
You can see if you have FileVault turned on by going to
System Preferences -> Security & Privacy -> FileVault (Tab)
Also I think that its important that we let Apple know this is going on. If you feel up to it I would send them feedback. I mean if you would like to that is. https://www.apple.com/feedback/macosx.html