I just thought about a thing...
1. We know that both Larasium and Atium are Preservation's and Ruin's "bodies", part of their souls, you can call it.
2. We know that hemalurgy is about stealing other's powers, and as Sazed said, part of their souls.
So, I wonder, that maybe Larasium and Atium would provide hemalurgic effects if the spikes aren't used to pierce someone's body. Because, they are already loaded! Loaded with Preservation's and Ruin's souls.
Or maybe it's like only Larasium can be used like this, because Ruin wanted his body only for himself, but Preservation gave his power to humanity, giving it up.
Also, I have a question. Since there is a balance, and there is never more Ruin than Preservation in total, does it mean that if atium comes back to earth after it's used, would Larasium do so as well? It takes hundred years for atium, and if larasium would, I think we have a storyline for the next possible book in Mistborn world, where there are only Mistings, and suddenly, the possibility of becoming a Mistborn reappears. How much chaos would that make and how people would react to it, it's a perfect background for a story of Mr. Sanderson.