That's a very valid question - It's not intuitive to create a 'fresnel' effect with something other than the 'fresnel' socket lol. It's my personal preference because Facing is set to a 0-1 range (white to black) by default as opposed to the grey-ish range of the fresnel. By using 0-1 it's easier to know exactly what values I'm swapping in. When they're grey, it's not quite so clear for fine tuning.
I sometimes use the Fresnel socket instead of Facing. I suppose if I always used 'Fresnel' someone would be bound to ask why not "Facing"...I guess the options are there for a reason; for preference sake.
A benefit of the Fresnel socket is if you want to be scientifically accurate, you can simply plug in the fresnel value of a material instead of 'guessing' till it looks nice.
