I feel bad for the Tauren. They still feel the obligation of thier debt to Thrall, yet they have to ally themselves with the Goblins and the Forsaken, both of whom commit actions the Tauren must find horrific.
I cannot say how the Alliance would treat the Tauren, but with Garrosh and his blatant disregard for the lives of Tauren warriors(as demonstrated in Tides of War) the Tauren certainly have valid reason to wonder if the Horde is their best option. Its sad how fractured the Horde is becoming. Maybe Warchief Vol'jin can turn things around.