If you use light cache for glossy rays, you may notice small artifacts in reflections which can be avoided. Although written for Cinema 4D, the same works for 3ds Max here: http://vrayc4d.com/manuals/faq/how-avoid-artefacts-use-light-cache-glossy-rays. It is known to reduce render times by quite a bit but it would depend on your scene.
Light cache does get baked into the irradiance map but if use light cache for glossy rays is ticked, everything but this setting gets baked which means you have to run the light cache the second time. You are rendering the light cache twice but in practice you are saving a lot of rendering time by using light cache for glossy rays, unless of course for your scene you don't experience any gain, which is unusual. you have to have a good amount of glossy materials in your scene obviously for it to be effective.