The LHT spoke holder holds 3 spokes. The DT requires 4 different spoke lengths for the 2 disc wheels. Either mount a spoke holder that works, or leave it off and knock a few bucks off the cost of the frameset, passing the savings along to the customer.

Fixing any braze-on to the frame compromises the tube strength in the heat-affected area to some extent. To do this for a gizmo that's mostly a marketing device is just a bad idea. Also, looks matter. A nice clean frame looks better to most people than one cluttered with gizmos and decals.

Even if the spoke holder held 4 appropriately sized spokes, it doesn't make much sense. You're far more likely to need a replacement inner tube, tire, derailleur cable, or disc brake pads than a spoke, provided your wheels were well designed and built properly. Where's the inner tube holder for my DT? Makes more sense than a spoke holder. How about a paper towel holder and soap dispenser? Now that would actually be useful...