It also takes some pretty sophisticated setups to even
measure .003" accurately. For example, if the rod in your dial indicator isn't perfectlly perpendicular to your fence/blade/whatever, you're gonna get another .006" or so of error.
Then there's the issue of the straightness of the materials used to construct your fence. On my Powermatic's Accufence for example, the factory BB panels that cover the rectangular tubing of the fence vary by probably five times that – .015" – in and out, like a sine wave, all along the length of the fence, due to the clamping pressure of the t-nuts used to fasten the BB to the tubing. But even if you stripped off the BB panels, unless you scrape and polish the steel tubing of the fence, a speck of rust is gonna quickly throw in another .010" or so.
If you add to that the fact that your wood is going to move maybe five or ten times that .003" every time you so much as breathe on it, you quickly realize that three mils has as much meaning in this discussion as three
Angstroms.
I'd be interested to see the woodwork done by those who think half the thickness of a human hair is "a lot."
This kind of thing gets silly after a while.