So, I've been thinking lately - maybe there's a reason,why the DH have two specs - maybe it's connected with Illidan's destiny?
If you have read the Illidan's novel,you know,that X'era has foreseen him a fate as the championg of the light,so maybe the third spec is currently locked and will become available in time - you know,when Illidan becomes a wielder of the the light, the Illidari would follow his lead and later could let you choose - to wield the fel or the light. Well,this is only speculation,share your thoughts!
P.S - before somebody starts screaming about the spoiler tag in title - yes,I better write it in,than listen to the shitstorm from those,who haven't read the novel or did Illidan's quest chain.