A perfect example of not wanted a snapping ruler: you are shoveling through a wall, and will soon break through it completely. After you break through, you will fall a good distance, and you want to know if that distance is splat height or not. A single pixel will make the difference between safety and death, so you pull out your trusty splat fall ruler to measure. However, you cannot get a measurement, because every time you move the ruler next to where the lix is going to shovel out, the ruler snaps to a different location. You try to move the ruler up to the proper place, but then it snaps far too far upwards, making it useless again. You are now stuck with a ruler that snaps to all the wrong places and you cannot possibly find out if the fall is survivable or not.
However, with a ruler that doesn't snap, you can move it to exactly the right place and get an accurate measurement of any fall every time.
Personally, I see absolutely no advantage to snapping the ruler at all. It bothers me that the NeoLemmix ruler locks in place after a second.