When I say my Swampers are nearly half worn, that's because the raised center chevrons are about half gone compared to my spare. I can run the tires for probably 20K miles after the raised siped area is entirely gone, they just won't have anywhere near as much traction, so I'll probably replace them well before that. I think that's what MT meant as well--i.e. "w/ 40K miles, I'm at half tread height, so I have xxx miles left on them" where xxx might be in the 10-15K range, not another 40K...
Or maybe I'm just confused and everyone else makes sense.