I have been wondering the same thing, as I am in the process of setting up my first tank. I asked this very question to the guy at one of our saltwater specialty lfs
's and his response was that these pieces would have the beneficial bacteria and possibly some coraline algae growth on the branches as they cure and absorb the nutrients from the water. Also, smaller fish would be able to use it as a protective cover, depending on the spread of the pieces. The coral wouldn't start growing, of course, but it wouldnt stay the same as it is on the shelf.
He said the downside was that the branches would also catch any floating debris if there were anything like large flakes or pieces of the filmy algae (forgot what he called it) that grows on flat surfaces (like glass or the plastic walls from the overflow) that break away for whatever reason and detract from the look of the tank.
Let us know what you decide and how it turns out.