ARC is nice, but in 99% of the applications it's completely unnecessary. Especially if you're shooting a modern bow with high speed (IBO near 300+). If I had an old bow and a choice of this or a new bow...it would be the bow every time! In this example, you're on relatively flat ground. The line of sight is the hypotenuse of this right triangle. From physics, we know that we aim based on the horizontal axis.
Before anyone gets worked up by this, let's do the math. The higher you are, the more the angle comes into play. For arguments sake...let's say that you're 30 feet up. I don't know very many who hunt this high, but it's an extreme example. Plus, it's easy math, as this is 10 yards.
Most hunters absolutely won't shoot beyond 30 yards for deer. This is because at that range, the scale tips against you. Beyond 30 yards, the deer can hear and react to the sound (even a quiet bow makes some noise) and jump the string before the arrow gets there...but that's another story.
So let's say 30 is our example here. You don't want to shoot beyond 30 yards. For arguments sake, the ground is relatively flat. If the deer is a horizontal distance of 30 yards...the line of sight is 31.62 yards. I don't know about you, but at that range, I use my 30 yard pin...
If you are a daredevil and decide to shoot at 40 yards...the line of sight is 41.23 - again, I don't have a 41.23 yard pin...
So if you're hunting some extreme topography along a cliff side or something, this gizmo may help you. If you like the rest of us are not hunting an extremely sloped hill, why pay for the extra feature that gives you 1 yard more accuracy for the horizontal axis?
If you're on a budget, I like the angle calculator...seems like that idea would work well...if you use different hanging stands at different heights, you'll have to be sure to reset the thing!