An interesting use for the Opacity value is for creating AGS (stands for Architectural Glass Solution). An AGS is a "sort" of glass material, very useful for window panes on architectural projects, where you don't need the time consuming properties of actual dielectric glass (refraction and caustics) but you want transparency and reflection.
This "trick" is created by using a single mirror material (white Refl0, high Nd around 20 or 30, and low Roughness around 0) but setting the opacity of that only layer to a low value, around 15-25%. This reduces the visibility of that material although presenting reflections, but reducing dramatically the render times as caustics and refraction are not calculated.
AGS glasses can also be tinted, of course, by adding the desired color on the Refl0 and Refl90 colot slot.
Using AGS is a good and fast rendering solution for window panes and glass doors, while you can leave real dielectric glass for jewelry, crystal objects and liquids.
You can quickly create both tinted and clear AGS materials using the material Wizards found in MXED, the material editor in Maxwell Studio or the plug-ins that have implemented the material Wizards.
Creating a AGS fast rendering glass by reducing the Opacity of a mirror material. You can download this material example here.
Find more AGS examples in the MXM Gallery.