Quartz is pretty plentiful in the Nether. I don't see much point in having a mob drop it.
I'm working on building a Japanese castle and it requires quite a bit of quartz. I could use concrete, and it's a great block, but the white concrete has a grey/blue undertone. Bone can be used, but it's a bit dingy. Wool is another possibility but the texture isn't smooth. Diorite would be okay if it didn't have the black and grey specks in it. In addition to it's innate beauty, quartz is the only white block that can be slabbed and staired. (I'm not counting sandstone since it's more cream/pale yellow.)
That's a weird reason for this.