Perhaps this is already a planned feature in future releases. With the upcoming material layers, a really nice workflow improvement would be treating entire material layers as changeable parameters in of themselves. In the same way that you can apply different material instances to meshes and see then update, it would be really nice to be able to change material layers in a material in realtime.
Then developers can author a M_Layer_Grass, M_Layer_Rocks, M_Layer_Undergrowth, etc and it will greatly improve the useability of Landscape materials and other complex materials.
This is a preferable workflow to the current one, as it means you can introduce layers with additional parameters and features (World Aligned textures, fuzzy shading) etc, without bloating the master material itself with tons of switches or unneccessary features.
Even if the underlying functionality was no different to static switches, requiring the shaders to be recompiled, it would massively optimize the process of authoring/customizing landscape materials, and make landscapes more modular.
Something along these lines 
