Portal lights is a feature mainly intended to focus background photons emission. Correct me if I am wrong but background lighting uses no attenuation for light decay when sampled for the direct lighting component, while when a portal light is sampled for direct lighting, it behaves like a normal area light so inverse square (quadratic) attenuation is used. The result is that portals don't really behave like a background lighting substitute which is its not the intended purpose. While background lighting produces constant shading regardless of the surface location, anything close to the portal light will become overexposed.
The feature request is to allow for no attenuation in portal light direct lighting sampling component and photon emission to first bounce segment in diffuse and caustic maps, at least as an option, so it really behaves like a background lighting substitute. Correct light decay management is very important for render engines to get good results.
There are other issues with portals, for instance when sampled for direct lighting, although they might have no decay, they will eventually produce soft shadows according to its actual size and not according to the background 'size' they are supplanting, but I don't know whether there is an easy solution for that.