I agree that a PA when the seat belt sign comes on is not required as the cabin crew do a visual check. However the visual check is no more than a quick walk through the cabin, I have seen on numerous occasions that they do not pick up belts that are not fastened. Another point is that even when the signs are on the cabin crew usually do not stop people from getting up. Try to get up on QANTAS and you will know about it.
As for the new PAs from the cockpit I really do not see how they convey and more information than our current procedures.