An successul H-1 status petion includes two parts: one is the petition for the alien to work for the employer(petitioner) and the other is the petition for the authorization of stay. There are time limts for both parts. Most of the time the time limits of the two parts are the same. Under some special conditions, UCSIS may approve the first part but deny the second part. If this happens, the alien must leave the country immediately and re-enter with a H visa to get an I-94 which authorizes the time for the alien to stay.
when you change your status in the US from other nonimmigration status to H-1, you normally get a 3-year limit on the I-94 form accompanying the approval notice. This means that you have both three-year limits for the petition to work for the employer and authorization of stay. The three-year limit is not affected by the fact that you depart/arrive the US for multiple times with the H visa as long as the original petition is still valid (you/employer do not violate the terms and condtions in the I-129 form)
Dual intent rule allows the H/L status holder to stay in those status when they are in the process of green card application. They are not out of status even after their GC applications are eventually denied as long they can maintain their H/L status. Obtaining AP and EAD is incidental to the filing of I-485 application. Everyone seems to agree that keeping AP/EAD without using it will not "affect" the H/L status. But some lawyers make a distinction that using them will somehow "affect" those status.
I am not sure how this distinction came out. But from USCIS's perspective, you are presumed to use AP/EAD when you file the applications for them. AP is a subsitute of visa. When you use an visa to enter US, you are "admitted"; when you use an AP to enter US, you are "paroled". AP can be granted for many different reasons, not just because of the pending I-485 application; a person who has never been to US can get AP to enter US for some medical treatments. Although the inspectors at the port of entry make no distictions on how you get the AP, USCIS does know and have the record on your H-1b status and I-485 application when granting the AP (you submit all documents when applying AP).
When the inspector grant you an I-94 and issue an one-year authorization of stay after you show him your AP, it does not mean that the new I-94 will cut short the validity of your original H-1 petition and push you out of your former H-1 status. You can think of this issue this way: getting AP and using it is an incidental benefit for the filing of I-485; if enjoying an incidental benefit from green card application will jeopadize the original H/L status, how can the dual intent rule protect the H/L green card applicants?
The instruction for I-131 says that H/L green card applicants DO NOT NEED to apply for AP, it does not say that H/L cannot apply or use (or use to jeopadize the H/L status) AP. Checking through the instructions for all immigration forms, you will find that for all critical issues, USCIS give some special "warnings" in the instructions. If using AP will jeopadize the H/L status, it is undeniably a critical issue; but we see no "warning" on this issue.
In the 2000 memo that some lawyers heavily rely on, USCIS said, " there may be legitimate reason to apply EAD........". As to AP, I think USCIS would also say "there may be legitimate reason for H/L to apply AP........." if similar question on AP came out. Getting back to US as soon as possible to resume the H-1b authorized employment is definitely a "legitimate reason" for getting and using AP.
Finally, I would like to point out the authority levels of statute, regulations, and memos. Memo is just the operation rule, it has the lowest authority. If the reading on the memo contradicts with regulations, this reading is wrong; if the reading on the regulations contradicts with the statute, this reading is wrong. Dual intent rule is provide by the statute, any readings on the memo/regulations that contradicts with the statute are wrong.