What is wrong with adding the ability to use shaders? If most of us the window managers that are composited and gpu-accelerated, why shouldn't the display manager be? Why should we restrict ourselves and theme designers? Display manager is the one first things a user sees it has to be attractive, IMHO.
Because the login manager is one of the things that should never fail.
That means it should not display corruption because some theme designer made a typo in a shader. Or crash. Or be unusable in a multitude of other ways.