Sry for late reply was busy today.
ok i try to explane it a bit better,excuse my poor english,its not my naitve language.
its is not the simple 3 chromatic IOR dispersion trick,i was talking about,but in the kind of.this means that if you want to combine different shader layer you can not do it this easy way,otherwise i dont would ask for a shader combine node.
A short small offtopic but related.I have allready made a Idea for a color input at the mix shader,for cases like the dispersion trick,with the color input you would only need 3 different IOR nodes-put into the color fac-done.btw lucas sayed that is would be possible ,because the color weights in the fac gets multiplyed anyway,so it could be easly replaced with a color fac.
here you can read the idea with the color fac at the mix shader
back to this topic,this idea was about combine quickly the RGB channels of shaders you want to combine
(like you do with color RGB combine)
here a example where a shader combine node would be very helpful,if not for many other custom node builds,you need layering ect.imagine you want to add another layer and so on.
of course this is mainly for custom node builds,for a simple plastic shader i would not ask for a different node.
here you can see a node group i have builded a few days ago,it should do the diffuse and specular workflow in the rows underneath the shader balls.
i could make it happen,you could make fancy stuff like this, colored specular maps are possible.
however,
here you can see the need for the shader combine node.
the specular layer is splitted with the “dispersion trick” (remember how useful a colored fac input would be?)
however the shader layer underneath,you ca not combine this way i have done.its works for its screenshot,but if you look closely to the underlying shader how there combined.you can not use addshader,because you need the mix,but you only need the RGB channels combined for each “RGB dispersion”
this is one of many situations,if you want to mix or add shader colors and so on together.a simple rgb channel shader combination would help in these cases.
i hope you can see the benefits for this.
look how this combine node would look like
and here with color input fac at mix shader ,would be even more usefull in this build,look how small the build gets.(7 nodes less if i count right)
edit,here another example the shaders combined via mix shader,the result is wrong.
Why?simple math.the high reflective green channel gets mixed by 50%,the otherchannels are black.the energy conservation is broken now.
and connected via add shader
(i have disconnected the underlaying black shader to get only the trichromatic factor values,and not get in the way for this example,would be make not difference,but to make it clear)
as you can see the true reflection factors are way higher peer channel,since this build using a schlick approxximation for metals.this would be the expected result as if a RGB channel shader combine would be used.i hope this makes it clear finally.
if i would use a white shader underneath you can guess,what would happen.now you can see the need for a RGB shader channel combine node ,and not to forget the usefulness of a color fac in the mixshader.
edit,last words at the end,
if you have mixed your shader with the 50%33% method before,dont you wondered why your shader gets so dull?thats the reason.if you dont use a trichromatic factor,the broken energy conservation is not that obviously.because you get 50%50% of a white Fresnel mix (its 1),then 33% and 66,6 (its 1)again.but the underlaying shader mix is wrong.and with the examples above you know why.
the reason for a RGB channel shader combine node is,simplisity and energy conservation.
same goes for a color fac in the mixshader.