Out of curiosity, what do you guys plan to use thin film for?
I'll just add this - for studio lighting environments it can really help to attenuate reflections on dark surfaces and make them more interesting. Also, I have to show metals that have some sort of heat treatment and it helps a lot since the coloring of the effect is realistic out of the box without having to tweak gradients to death.
Here's my +100 for a mapping that's more controllable and consistent, and I'd even support going a step further and suggest real thickness units. It's just so much more convenient and practical to know 'mapping 450-650 nm looked good in my last project, let's just use those values and work from there' than some greyscale map that you plug into a CC node, tweaking levels/gamma until you get the right nm range without ever knowing what the mapped final value is.