Its poor practice - along with 'Urgent Matters' and any other non specific agenda item. To quote from the NALC - Good Councillor Guide -
'Councils must also publish meeting agendas which are as full and informative as possible'
I have been trained that Agenda Items should be written at proposals so that everyone is aware and clear on the agenda item.
The whole point of transparency is to be clear and open on the business that is to be debated. How can a member of the public decide to join a meeting that says AOB. We do not allow any decisons or discussion to be taken on any item not on the agenda.