um, i have to disagree.  there are lots of things i've come across because i've seen them be wrong in the previews.  i might try about 20 different combinations of parameters in different nodes, just tweaking off the previews (yeah, i tweak a lot).  even when i test render, i can adjust off of the previous preview and how it rendered.   if i just used Matmatic, i'd have to wait for a render each time i adjusted my parameters, or at the very least go through several steps just to see the results in the material room.  i'd still be on my first hair shader instead of whatever ungodly number i have now.  conversely, if i tried to base my hair shaders on pure math, i'm pretty sure i wouldn't get even vaguely close to what i wanted (as opposed to now, when i'm in the same neighborhood). because each time i change setting, lighting, character and intent, i find i need to change a lot of settings.   same goes for a lot of the adjustments i put into my version of the PR2 (soon to be PR3) skin shader.  sure, i could just put in numbers, but that wouldn't get me the precise skin attributes i want.  i have to see what i'm doing (how pale or dark, how much sss, what color, where, etc.).  GUIs are a good thing in general.

i'm not saying going the pure math route doesn't have it's place, or that Matmatic isn't a wonderful tool.   but if you got the best results by taking a purely command line approah to art, then there wouldn't be any interfaces on CG software.  professionals use tools with GUIs because it's time efficient and produces good results.  they combine their use with scripts for the same reason.