At one point I cancelled my Miner to make another profession, one where being level 90 made a difference. A month later I missed Mining (or I missed buying Ghost Iron Ore when that was 45% cheaper than buying the Bars) so levelled a new one from 1 to 525 on a level 77 toon.
I found that at many different levels I could use smelting, rather than going out in the world and farming stuff. Not 100% of the way, but at least 3/4 of my skillups were from smelting of Ore that I bought in the AH.
It was a bit costly -- ever since Jewelcrafting came out, most Ore costs more than the Bars you make it into, so smelting of stuff you buy usually loses money. And because so many players are 90 compared to 30 or 40 or 60, lower level ores often cost as much as, or more than, the higher ones. But for someone with several 600 crafters, it is affordable. Much cheaper than levelling BS or another craft professions 1 to 600.