Being able to choose a Custom Selection set of textures from among your purchased texture packs is a feature a lot of people I speak with agree is much wanted. You'll sell more texture packs to people as they buy packs they otherwise wouldn't use just to get the lava texture they like.
I bought most of the texture packs on 360 but none so far on PS4, as I found every one of the existing packs has a deal-breaking flaw. I'd most likely play in Natural but the smooth stone texture ruins it, but if I could use the smooth stone from Fantasy it would be another thing entirely. Some packs have better looking water and lava, and the obsidian from City is the best if memory serves.
Basically, if this were implemented I'd go from someone buying no texture packs to someone buying ALL of them, just to get a single block sometimes for the perfect set.
From 4Jangs perspective, I think they'd sell that many more texture packs as people would want all of them to mix around. The fact I'm still using the vanilla textures (as the only 'perfect' texture pack) is pretty absurd. I'd certainly buy Fantasy just for its stone. texture and put that in Natural, with maybe a few blocks from City.
I've been using the City Elements pack because I use a lot of glowstone, and the plain white texture of it is awesome, instead of the gawdy patterns in all the other packs. Oh, and the smooth stone is flippin awesome. But there are textures from other packs that I really like better than some of the ones in the City Elements pack for other things, so being able to pick and choose from the whole owned collection would be incredibly awesome.