NALCs rationale behind the change of wording is, I believe, on the grounds that the agenda is signed by the Proper Officer and therefore the Proper Officer is legally responsible for the agenda contents. When I was a clerk I rarely rejected a request for an agenda item but did draw the line at wasting councillors time on items that were outside of the remit of a parish council to consider or take action on, lacked sufficient information to be clear what was being required (in which case the item would be resubmitted in a clearer format, possibly for a later meeting) or had been received too late for inclusion on the agenda once published. Now I'm a councillor I still support that view as I don't want to be wasting my time on fruitless discussions on matters which are of no relevance.
There is, of course, a way around this. Your standing orders should set out the procedure to be followed to convene an extraordinary meeting. If you request a meeting for a specific item and your chair refused to convene the meeting, then assuming you have the relevant support form your fellow councillors, you can convene the meeting provided you give the appropriate notice etc. etc.