I didn't believe in additives either until recently. I have a 200g. mixed reef, mostly sps. I do have about 5 large softies, devils hand, sinularia, 2 toadstools and one finger type softy. They were all very large and doing well. They had actually grown to there fullest potential. They got very large and then stayed the same size for over a year. Just for kicks I bought a bottle of Seachem reef complete -amino vitamin supplement. I bought it because Tracy Gray(tank of the month a few years ago and in the book Ultimate Marine Aquariums), said that he had good results with it for his softies. I started adding it to my tank and I kid you not, my already huge softies grew about another 15-20% in under 2 months. I couldn't believe it. Non of my other corals changed a bit but the softies loved it.
Just my experience. Other than that, don't believe in additives.