Q1 The simplicity of having a single commercial type (a la BS 5839-1) system throughout the premises, instead of a mixed system with a communal fire alarm system/ domestic smoke alarms. Simpler to maintain, test etc.
Q2 Normally the ARC does not get the addressable info, so don't know what flat the signal is coming from. They simply get a single signal saying its Happy Valley Housing Scheme. How can they filter? there is no one to talk to and no means of communicating with anybody anyway. The caveat "normally" is inlcuded because you could do it (albeit not very elegantly) by putting an addressable relay in each flat and connecting that to the social alarm system. They then get the signal and can filter, in which case no need for the flat detectors to trigger the communal system ARC tx.
Q3 See question 1. It is a free country (primarily because comrade Corbyn is about to be discarded to the same sheltered housing as Michael Foot where every day will be a Saints day holiday). If people want to do things a different way, a BS should not stop them so long as the objective is achieved.