Most of the reasons I've seen, and have had some experience with, are:

David has beeter joints and is easier to pose. David is proportioned better in the body and face and is easier to sculpt into a more realistic (average) characters. He can share some morphs with V3 (such as She-Freak 2). Luke and Hiro fall into the same category, all being of slightly later generation. Luke doesn't have as dense a mesh, nor amount of morphs as David or Hiro, however.

If you use David, and also have Hiro (who becomes realistic with ease), they can share some clothing and, most importantly, the Hitoro character by Capsces, which is a new cr2 for Hiro (with a freebie David version at her site) loaded with a ton of extra morphs to get the most out of each figure.

That's just from my humble experience, as well as what some clothing creators have mentioned.

-- Jeff