Interesting question, I was wondering the same thing recently. For certain plants, I know they grow very well under MH, but create much thicker/nicer "flower blooms" under sodium lamps. So I was thinking it they may aid in blooming or polyp extension or somethign like that. But I know little about reefing, and since I never see any sodium talk or related hobby products, I assume someone has already proven them less beneficial than the halides.