There is a good argument in my opinion - to equalize the opportunities of all collecting professions. Skinners have a chance to skin high lv mobs and get Thick and Rugged leathers (and thick and rugged hides), miners have a chance to mine thorium and rich thorium veins and get ores/stones/gems without arcane crystals. Only herbalists are at a disadvantage in this respect. Allowing herbalists to gather Dreamfoill, Mountain Silversage, Plaguebloom, and Icecap will equal all gathering professions. In order to maintain balance, the black lotus must simply remain blocked (similar to arcane crystal block).