Walls are useful.

Any block which already has stairs and slabs should also have walls:

Quartz variants, copper, prismarine variants, polished diorite/andesite/granite, ...

Walls are insanely useful for building:

horizontal beams, pointy roofs, layered walls, detailing, pillars, thinner walls, ....

So many blocks got their wall variant, yet inexplicably, often used blocks, like Quartz, did not. I urge you to change that.