Except that it isn't. Looking at my logs I see that the PtL buff comes like 100ms after the last missile is cast (I have around 50ms latency so that seems to indicate client->server->client delay). So if I would cast the next AB exactly when the last missile leaves my hands, I won't have the buff yet. Since it's the client that determines spellcast duration, the client won't apply the haste to the next AB until it hears from the server that it has received the information that the last missile from MBAM has been fired.
The point is that the red zone on Quartz shows when you can cast your next spell without receiving the message that you are doing something else. For the special case of MBAM+PtL it instead needs delay showing the redzone by 2 x ping. Because the timing is so exact and Quartz doesn't show it, many players maybe could probably do better dps with nochanneling than to try to time it themselves.
2 different topics. Read the first line in my post.