Well if a NDB was co-located with a TACAN, it would let civilian aircraft get more than just a DME from that TACAN... So that could have been an idea... All up to the point when they began spreading of the VOR/DME's all over.
A NDB/DME could mabye be a poor man's (or poor airports') VOR/DME but with needle and no CDI. But then, the GPS came. At least in the US it seems like remote fields typically either have ONE NDB approach to the field and one GPS approach to each of the runways...