As you know, since the Burning Crusade expansion, the planet, Azeroth, is under constant threat of total destruction. So constant in fact, the so-called threats now feel more than forced, the story almost always focuses on a new global crisis every few years. But again, personal stories, interactions between completely different species or political struggles within factions may not give the "epic" feel a massively multiplayer game needs to introduce to it's audience.
However, choosing the global crisis path in almost every opportunity, yet not even mentioning the various sentient cultures of the planet aside from two major superpowers, Alliance and the Horde, causes some harm to the scale of the story. Leaving that much to "head-canon" is never good writing, and I believe at least some of the previously seen capable cultures on the planet should play a role, even if it's only little, in every new grand scale story that hits or threatens Azeroth as a whole.
Take the Tol'vir for instance,
After the Neferset was defeated, Ramkahen now controls most of the Tol'vir settlements in Uldum. They have an advanced, Egyptian-like culture, a political system, they have disciplined military, they are unique and are in control of one of the most distinct, vast areas of Kalimdor. Everything about these guys yell "Hey, look at me dammit, I should be a major player".
What about the Centaur? Sworn enemies of Tauren?
Originating from nomadic cultures, such as Mongols, Turks, Huns, these guys roam the plains of Kalimdor in great numbers. Such great in fact, they almost brought the whole Tauren civilization to extinction.
Centaur mostly lack "intelligence" to coopetare with other sentient species, they are bloodthirsty pillagers, rapists, raiders. However, they are also capable of keeping an eccentric nomad culture, a shamanistic religion and lifestyle that resembles early Turkestan and Mongolia closely. It is necessary to remember that the early eastern nomads were almost always seen as "the seeds of the devil" by the settled cultures as well, yet their agressive behavior and war-like mentality was mostly produced by adapting to the environment they lived, cold and harsh steppes, impossible to farm or settle, always living under the constant threat of starvation or getting their already few supplies stolen by rival tribes, wild animals, organized military of settled cultures trying to drive them away from their new-found lands, these people were almost always on the run for their lives, it's not surprising that they learned to return the favor.
Centaur may not be the exact copy of Asian nomads, but their settlements, culture certainly show promise and resemblence. If not anything else, they are an incredibly numerous cavalry force and cannon fodder to direct against a mutual threat, such as Burning Blade.
We also have Tuskarr, Furbolg, Yaungol, Taunka, Dragonkin, Elementals, Ethereals, Ogres, Dryads, Hozen, Jinyu, Vrykul, Earthen, neutral Goblin cartels, neutral Pandaren, Trolls, Gnolls, Quilboars and so on. Some in the list are much more suited for interacting with sentient species than others due to such factors as intelligence or instincts, some may have severe limitations such as not being able to leave a spesific area. However, I'm having a hard time to comprehend how and why they leave everything to "most trusted" Alliance and Horde to be dealt with, building bunkers to hide in, hoping the danger just flies away.
TL;DR
Writers have created many cultures and civilizations to interact with each other, especially during times of crisis. I believe they shouldn't have let their only part in the story to be "go kill 10 of the x race for me then forget that I ever existed". It would be a good, may be even necessary touch for new storylines to include at least some of them, even they are just minor roles.