If your blind, you use either braille or text-to-speech (usually braille, I'd guess), if you're deaf... well, you can still see. If you're both deaf and blind, then braille is the solution for you. If you're color-blind and deaf... you can still see, and you can just switch the theme. If you're visually impaired and deaf, you can either use text-to-speech or braille (or a high-contrast theme)...but I don't think you'd be in much better shape with GDM either.

Now, could you name a set of these conditions that might occur at the same time which would prevent someone from using LightDM and not prevent them from using GDM?

As far as one person needing one, and another person needing another, I addressed that, no?