That's a little silly. The spoke holder is for carrying spare spokes. If you don't pay attention to it until you're on the road, then of course it will be useless. You forgot to pack spare spokes or else you would have figured out if the spoke holder worked before you got to Bear and Lion country.

It's designed to take 3 spokes, so in theory it would accommodate a spare spoke of each size. In practice, I currently have only one spoke in mine, a front wheel spoke. The spoke holder is made for what I'm sure is a standard range of 700c spoke lengths, but the spokes used on my fat, IGH hub are too short for the holder. I just taped a couple of spares to one leg of my rear rack. So, yes, the spoke holder may be something of a gimmick, but it's also perfectly functional for a certain length of spoke, but you won't be learning that on the road. You'll learn it when you pack. What you'll learn on the road is whether or not you remembered to pack spare spokes.

It may be, as Stannian says, that many wheelsets are made using only 2 spoke sets, but if you're not using a pre-made wheel set, you could easily end up with three spoke lengths.

And as Werkin points out, we're talking disc breaks, and disc front hubs are dished. This has potential for you two have 2 spoke lengths in the front, and two different spoke lengths in the rear, so you still could potentially run out of room on your spoke holder. But I imagine that for most riders it is sufficient, but it's such an inconsequential piece of the bike, it hardly matters. It doesn't suit my spoke needs, but I lost no sleep over it.

Also I've heard it suggested that moving the spoke holder to the drive side would let your spare spokes double as chainstay protector and would make it easier to mount a rear-mounted kickstand. The disc-trucker might address the first issue, but it looks like the disc brake mounting assembly will still interfere with some kickstands.