Hi Tim,
Rendering transparency with OpenGL is not as simple as managing it with ray tracing, where it is naturally done by the method.
In OpenGL, transparency is processed by sort of compositing between surfaces. So quality is highly dependent on surface order, and there is often drawbacks induced by crossing surfaces.
There is some methods to render transparency correctly ("depth peeling") or roughly ("faces sorting"), but they are not efficient because they are view dependent, and need to update every frame.
I suppose that Martin want to preserve efficiency and reactivity of the editor.
Cyril.