The 25" long FPC locking extension doesn't have the hex machined to it, unlike my 5" wobble extension. I just looked through their web catalogue and notieced that the hex feature is only available on their wobble-only extensions.
For fast hand spinning, the more cylindrical the shape is the more comfortable and faster it is to spin. With grip and speed in mind, I think the Ko-ken style knurling or the ones from MAC tools or Wera, for example, are better.
As a feature, I think the hex machining will likely come into greater use on a road kit, where you might have limited tools on deck (ie. not having the luxury of taking XL extensions with you on the road or say, for your home box (as opposed to your work box)). In such a case, you can couple it with a 17mm deep socket and extension to increase the overall length a little more, if needed. That socket could be a 1/2 drive or a 3/8 drive, depending on what you have.
Of course, the main reason (according to the manufacturers that make these types of extensions) is to be able to drive the hex with a wrench. Again, that feature would probably come into use more frequently when your tool arsenal is limited.
I personally have not encountered a situation where I needed to use the hex end. If the need ever arises, it's likely going to be for some sparkplug R&R. Who knows...it's a feature that's there, and may come in handy one day.
