The alternative to splitting nodes would be to set a higher ambient color on the Phong materials if you are mixing.
If it comes up too often then maybe we can add a boolean to turn it on/off… But I have to say that when I first tried to get me PBR scene to look right the first thing I did was just add an ambient light. I was disappointed. 
Why would one mix phong with PBR? When I made the switch to PBR I accepted that I’d have to translate all my phong shaders, I never really thought mixing modes like that was an option because it would be really obvious.
I ask out of curiosity more than anything. I can’t think of a case where it would be visually appropriate.
one example. JME terrain can be only phong(where anyway even normalmaps dont work), unless you modify it yourself(i tried with help, but had problems so i skip it for now)
and again, im fine with it because i have separated nodes anyway, but i think about new members or anyone who will be surprised about mixing it.