Many people wonder why the Horde stays together even though the goals of the different races are also as different as said races. I think Key can be found in Christie Goldens book Rise of the Horde
Quoting Thrall on page 99 start of Charcter 6
The Lessons from that time were bitter, bought with blood and death and torment. But ironically, the thing that nearly destroyed us was the thing that would redeem us later: a sense of unity. Each Clan was loyal to itself, fiercely dedicated to its members, but not to the others. What we united under, and against was dreadfully wrong, and for that, we are atoning still. Generations after me will still pay for those mistakes. But the unity itself was glorious. And it is that lesson I wish to recover from the ashes. It is that lesson that caused me to speak with the leaders of so many seemingly different peoples, to work together towards goals we can all be proud of.
Unity. Harmony. That is good lesson of the past. I have learned it well.
What this shows in my mind. Is that Thrall knows full well that undead and orcs don't agree on a great many things but like the Orc clans of old they have enough in common that they can get a lot. But more than that he wishes to make all the races of the Horde see that in unity they are a much best and stronger group than any one race alone. Even the Orcs.
But sometimes you don't realize how good you have it. Until you lose it. I think part of the reason to put Garrosh Hellscream in charge of the Horde is not only to increase the conflict but also set up the return of Thrall. Only this time the Orcs will understand what Thrall brings them.