It's an extra step that gets in the way in the overall process.
I remember this product / topic came up when I was in make-up school in the mid 90's, and even today I still agree with the instructors' take on these... These are a crutch. It reinforces sloppy, unrefined work. If a person can't stay within the lines or prevent major product *fallout, then that person needs to practice more, modify their technique, or change up their product.
The only time you use any type of shield, is when you airbrush.
* Yes, some product will fall, but not enough to warrant using one of these. Some already have mentioned alternative methods they use that work.