As with anything else, the smelter is very useful, the player just needs to prioritize. Brick making has the same problem. Particularly if you like to tunnel, you wind up with a ton of rubble to either pack into bigger and bigger stockpiles, or turn into brick and sell in bulk, however if you focus too much on it in the early game, you're going to find yourself short in more important areas. You have to know what and when to prioritize.
As for the furnace, I don't know where I'd put all of the bodies without it! You can only put graves in arable soil, which is a waste, and I rarely have enough incendiaries lying around to cremate them that way. The only real problem I have with it is that it uses power (I'd rather it burned wood, honestly), but I just turn it off when it's not in use, like turrets and A/C in the winter.
As for the furnace, I don't know where I'd put all of the bodies without it! You can only put graves in arable soil, which is a waste, and I rarely have enough incendiaries lying around to cremate them that way. The only real problem I have with it is that it uses power (I'd rather it burned wood, honestly), but I just turn it off when it's not in use, like turrets and A/C in the winter.