I know many people are using the physical_light. Usually they use it with the intention, well, to obtain physical correct renderings. I'm sure most of them don't know that sometimes they put the fox in charge of the hen house this way. Why this?

First, physical_light does cause problems when used together with Maya standard shaders, like Maya blinn for example. Here's a rendering with raytracing set to on, the material is the default Maya lambert, the light is a physical_light with intensity 200000 attached to a Maya pointlight:

Looks good so far, but what happens when raytracing is turned off? Here's an example: