it’s still a crafting reagent, so unless Blizzard plans to remove it from the crafts it’s used in it won’t get turned into gold - or a currency.
Would be nice if they made it not soulbound.
I have found an “exploit” of turning it into gold. As Jewelcrafter I create a profession equipment that vendors for 50g, but the cost of the materials at the current market rate is close to 15g. So 15g + 225 mettle = 50g => 225 mettle “converted into” 35g. I suppose other professions may have something similar. Gathering professions can achieve the same through crafting orders on an alt/friend, but at reduced profit of course.
-=EDIT=-
Not to mention that multicraft and resourcefulness procs reduce the cost.
I’m still keeping some stacks in the bank, but I have just been deleting any new ones for the past 2 months or so. My bank was really suffering before the Warband bank kicked in!