My experience has been that on occasion flight attendants use a late departure as a reason not to do a PDB. Personally, the way the aisles clog up in the back whilst people stow the carry-on bags, I don't think the PDB is the problem. It is all the carry-on bags that delay everyone getting seated and out.

My personal opinion is that the PDB is both a tradition and an essential requirement for the front cabin. If we can take five minutes to explain to everyone how to buckle a seat belt, we can serve a PDB.