When you get a lot of materials right-clicking to delete materials is both slow and tedious if it even works. Why not have a button for deleting materials just like you have for layers. This is different from purge, rather its for materials that are being used. The option to replace with another material or default material would be nice as well.
To quickly remove all materials in the model you can use this extension: http://extensions.sketchup.com/en/content/material-tools