The agitator for my Sitrex is one of many of my 'just in case' tools. When I purchased my spreader, it also came without the agitator. I asked at the dealer why and they said basically the same thing. The agitator will wear away the paint on the inside of the cone and create a rust line where it touches the cone. Over time the rust will wear through and you will end up cutting the cone in half. When the salesman told me this, I thought of the spreader at the farm where we used to board our horses and remembered the rusty groove they had in theirs.
So I bought the agitator, 'just in case', but have never installed it. I have never needed to spread anything that was a powder. Everything I spread is in granular form, and unless my fert. or lime is wet, it drops right through the cone.
So the agitator sits in the original box. On the shelf. 'Just in case'.