That's sort of presumptuous Mass, I could argue fire for warmth, air to breathe, earth for crops. A major theme of the elements in Warcraft is that all of them are vital to life.
Earth has nothing to do with the purity of one's blood, but the purity and integrity of their goals and motivations. Its saying that "this is a noble goal, one I can dedicate myself to fully", the option explicitly mentions the purified, and the Mag'har have moved passed discriminating against greenskins. it's not like Green orcs can't wield the elements in WOW. Earth is all about dedication and conviction, and that sort of conviction more than anything else is what drives Dranosh.
Dranosh fights against evil in all the forms he come across. Be it with word, sword, or courage. The fact is the whole point of the Mag'har is to have a society fully devoted to confronting evil, be it their brethren, the legion, or anyone else. Unifying the orcs is only a small part of that, and done explicitly to prevent them from losing sight of who the real enemies are.