Why do Proctor and Rebecca speak out against Hale's coming?
I don't believe the text tells us why directly, but certainly lets us infer why as the play proceeds. Initially, it seems that both John Proctor and Rebecca Nurse do not believe that these issues being discussed regarding Betty Parris' recent struggle with her health have much to do with witchery, but more with hype and raising havoc.
Rebecca believes calling on Reverend Hale will just begin again much division in the town... it would bring about accusations of who is witched and who isn't. Rebecca is the voice of reason as she insists that yes, relying on a good doctor and strong prayer is a valid path to recovery for this young girl.
Proctor is also reading between the words of the Putnams. He sees that they are trying to influence the new Rev. Parris and he thinks they are using their hefty land ownership to do it. Proctor also notes that Rev. Parris himself seems to be swaying of late.
Hale is an alarmist who brings with him great "knowledge". At this point, this sweet little town needs some common sense and both of these folks recognize it.