When should directional complements be used? How come there are many times that they aren't used as in "他们要到上海了"? I understand that Chinese directional complements are used to indicate direction of an action. I understand that when 来 is used after the verb there is some movement towards the speaker and when 去 is used there is some movement away from the speaker.
However, my question is when is the directional complement used because I see sentences in which it could be said that there is movement to/away from the speaker and yet there is no directional complement.
For instance:

他们要到上海了。(Tomorrow they will arrive at Shanghai.)

When are directional complements used? Please answer generally rather than just answering in the case of the above example.
Let me put it in this way:

他们要到上海了。 
They are about to (要) arrive in Shanghai.

This sentence means &quot;they&quot; are already on their way to Shanghai, instead of meaning they are going to Shanghai or they are planning to go to Shanghai.
Moreover, this is just a statement of fact, and it doesn't require/imply any relationship between the speaker and the object (&quot;They&quot; in this sentence). The speaker can be in Shanghai (The speaker is telling his friend that those people &quot;they&quot; are about to arrive here); the speaker can also be somewhere else.

他们要到上海来了。
他们要来上海了。
They are coming to Shanghai.

This means the speaker is in Shanghai.

他们要到上海去了。
他们要去上海了。
They are going to Shanghai.

This implies the speaker is not in Shanghai.