There is no what here, absolutely top use for Hand of Guldan was to triple Chaos Wave burst into adds, of which in BRF you had plenty. What you say is true only for single target, it's not where the real damage was.
No adds there lived long enough to chain the HoG dot on them to make it better than nuking them down with Chaos Wave. The only exception was those 3 bitches and even then on pull you wanted to nuke them down while they were stack before they split up.