Honestly . . . it falls into the realm of carbon water bottle cages. It is not an absolute necessary to own before having an enjoyable cycling experience, but that said, I have them on all my bikes. The biggest advantage is that, as you state, it puts the GPS unit further out in front of you so that you don't have to look down so much to see it. I have a K-Edge and several Bar Fly mounts. (That's a good price on the K-Edge that you posted.) The only thing that I don't like about the K-Edge, (and is actually an advantage from different viewpoint), is that being AL, it could scratch the paint on my handlebar when I torque it down. What I do is cut up an old inner tube and wrap the bar once around before screwing down the mount. Works well enough and I don't have to worry about scratching. The Bar Fly is a plastic composite and will not scratch bars.

I'm just curious about your rubber band Garmin mount. Why do you have it on your handlebar? I would think that for sure it would not stay in place because the bars are smaller in diameter than the stem. When I used the rubber band mount, I always mounted it on the stem. Never turned in that position.

p.s. I do have carbon water bottle cages on my bikes. I'm partial to the Blackburn cages.