I filed cutters free hand for years, but always used a gauge for rakers, unless in a pinch. I thought it was real good and it was for the most part, but using the simple guides is so much more consistent and when it comes down to it, easier, why not use them?
I never met a tree guy yet that didn't think he was really a good filer, yet I have not seen a good filing job by any of them. What does that tell you?
The most important part is smooth, consistent cutting. Most guys think if it grabs like heck, that's good, but it should cut smooth and fast, grab and pull sucks.
My point is that you can adjust the gauge to cut to your parameters, and then every cutter and raker is exactly the same. If you file free hand, it will be inconsistent, no matter what kind of robot you think you are.