Emergency descent after depressurisation is a good example of when a MAYDAY broadcast serves a vital purpose in alerting other pilots to listen carefully to the message which follows, as they may need to take avoiding action and be prepared for some rapidly changing TCAS advisories. There's not enough time in such situations for ATC to assimilate the emergency message, assess its consequences for other aircraft and transmit bespoke advisories to each affected callsign. The idea from our nautical interloper that aircraft are 'on their own' and don't stand to benefit from others being immediately aware is rather crass!