Alright, let's look at what SIVA is. In a nutshell they are little nanobots that rapidly build, SIVA is very advance as it could also provide power, build armor, build onto weapons, and modify existing technology. It can revive and repair machines endlessly. Now let's look at spin metal. Spin metal is liquid metal with nanobots in them used to rapidly form solid metal structures. It's not as advanced as SIVA is as all it can do is build. This sponmetal is placed in containers that overtime break and this spinmetal leaks. It is used in armor and weapon because once it's damaged it can repair itself up to a certain point... sounds familiar? It's essentially nanobots that build very rapidly, similar to SIVA. However we can't use it like SIVA as it can't do all the fancy things SIVA does. It's almost like a prototype. If spinmetal is controlled by some nanobots, these boys have to have some type of command system right? You have to be able to tell these bots what to build.. does someone in the vanguard control it? Does the user have some device that controls it? What happens if someone uses this for evil? This stuff is infused in your guns and armor. If someone could control this spinmetal and they had evil intentions, they can create an all out guardian purge. Imagine casually patrolling your favorite planet when all a sudden your armor and gun starts rapidly spewing liquid metal that hardens, engulfing your entire body in a metal prison. You can pull some order 66 type of shit with spinmetal.