Actually, demonology is special in that reguard - they can summon a special demon that is more powerful than the base demons that the other two specs can summon.
Demon Hunters have two specs because they did not want to have two dps specs that are redundant for each other, and use the same gear, etc, and force players to play the one that yields higher dps anyway (like Frost vs. Unholy).
Demon Hunters can only be two races because the lore story behind the demon hunters we'll be able to play as are Illidari - the demon hunters that Illidan trained at the Black Temple before his defeat. He on