I can't speak for anyone else on the matter, but I like to buy Dead Orbit gear for my characters to have armor with a black color scheme. Still haven't gotten a shader from their packages. Not to mention that, at least in my opinion, the faction gear tends to look a lot better than the Vanguard gear. Not to mention that some people (not me, but I know a few of my friends do) try to indulge themselves in the game world as much as possible and support different factions by wearing their marks/cloaks/bonds and armor. Sort of like a roleplay thing, I guess, I wouldn't know. I don't see any downsides to making it possible to buy faction gear with Vanguard marks.