Suppose you are a teacher of Buddhism in a place (such as the United States) where there are many forms of Buddhism being taught. Your students may have "shopped around" and could be asking, for any particular subject you might present, why such and such other form of Buddhism says something else on the subject. There might even be a contradiction. What do you say to the students, that the other form is wrong? that they should not think about it right now and learn in just the context you are presenting?
Suppose you are a student of Buddhism. You have been to several, even many, teachers of different schools of Buddhism. Some of what they say seems to be common, some even opposite. Even some of what is common is presented in such a different way it is difficult to reconcile the two teachings. Will you become a student of comparative religion? Will you give up on Buddhism, which might seem to be only nominally Buddhism but actually very different teachings each using the label "Buddhist"?
These seem to be the kind of issues Joseph Goldstein is addressing in "One Dharma". It is an appeal to recognize:
1) what is common within Buddhism (and that as very important)
2) not a tolerance but an appreciation for different ways of expressing the teachings.
3) an acceptance of philosophies that conflict so long as their associated practices work.