In the situation you quote (smoke in cockpit), unfortunately POB is something the Airport Fire Service do need to know, and it might literally be a matter of life and death. There have been too many tragic accidents over the years, when the aircraft has landed and stopped intact, yet people have died in the ensuing/continuing fire. As much as we all hope it never happens again, the AFS have to know how many they're looking for if they have to go in.
I've yet to work at an airport when I could be confident that the handling company could be contacted and give me, within a couple of minutes, the exact and final POB figure.
I know nothing of the DUB incident, but strongly doubt the controller was either trying to p*** the crew off or make idle conversation!