Yeah but isn't it a bit quicker to say 'Qantas 415' than 'Tango Uniform Juliet'?
That's debatable - depends on the callsign (Mike Mike Mike?) and the airline name adopted by other users. I'd rather stick with TUJ all day than 4 or more FNCS changes in just an afternoon's flying.
Then there's the abundance of numbers when you add 3 or 4 more to a call already heavily populated by numbers - 'QF### at ####ft turn left onto heading ###, climb to FL### ... traffic ahead is QF###'.
And if they're not sure, won't ATC usually ask what type and how many POB when you enter their FIR/ area?
I should hope so, but the rest of us locals on a visual approach and looking out for QF415 don't know if we're slotting in behind a B747, B767, B737, B717, BAe146 or a Dash 8 and would have to ask if not told. Rego's become familiar and we can use this info to confirm what ATC have told us - FNCS's change at a whim and offer little help to most at the coal face - QF1 excepted.