So either switch will pop the RAT out, but only the ELEC will force the RAT Gen to power its respective systems. It looks like this is the reason for two switches; the RAT Gen takes priority over the systems it feeds.
So, having a HYD failure, pressing the HYD button will get the RAT out without force powering the RAT AC Gen. If you had a HYD failure and still had serviceable engine generators, but pressed the ELEC switch, you'd pop the RAT, still get hydraulics but you'd also force off the good engine generators (only on those bus bars the CSM/G supplies of course).
If you dispatch with APU GEN inop, and have an engine failure in flight, you're left with only one electrical source just after the failure has occurred. Should there be some form of electrical generation malfunction with the remaining electrical source, would you enter mechanical backup law whilst on batteries during the time taken as the RAT auto deploys and EMER GEN couples to the electrical network? Or would you remain in alternate law (gear up of course)?
If mechanical backup law is true, would there be a case for deploying the EMER GEN if you were reduced to a single electrical source in flight? An electrical malfunction at an ill timed moment could leave you in no man's land whilst the RAT deploys and generator is coupled. Would the EMER GEN just power the essential systems as per ELEC EMER CONFIG and leave the less essential systems to be powered by the remaining IDG? Or would it be more complex?