First we need to understand the difference between H1B visa and H1B status. We get our visa in US Embassy or Consulate. It’s an one-time concept. After we enter the US, we talk about our status. People can keep their previous status as indicated on their passport, they can also change to another status. If people enter the US on H1B visa, they can freely travel outside and return back to US within the valid time slot. This is just like we can travel to China and return back to US without re-applying for visa within six months when we enter the US on F-1 visa. However, most of us here enter the US on F-1 student visa or J-1 visiting scholar visa, and we usually change to H1B status which is processed by INS after we get a job. Because we make this change in the US, it is valid only if we stay in the US. If we travel overseas, we automatically lose our H-1B status. At this point, if we want to re-enter the US, we must re-apply for visa. This is the key point as to what the difference between the H1B visa and H-1B status.
Second, we get our AP and EAD when we apply for I140/I-485. If we travel to China, we automatically lose the H-1B status, but we can still use AP to re-enter the US, this is just what AP is for. At this point, our status is I-485 pending, not H-1B, but we can work with our EAD and freely switch to another employer. Of course, we can still apply for H-1B visa in US Embassy or Consulate in China, and use that new H-1B visa to re-enter the US. At this point, we have H-1B status as well as I-485 pending status.
Third, if we use AP to re-enter the US, the worst case is that if the I-485 is rejected, we lose all the legal status, and we must leave the US. So the advantage of having both EAD and H-1B status is to avoid this worst case. This is the major reason why most of us having AP choose not to travel to China until we finally get the GC.