I think those sort of events with forgotten sponsors are part of a much bigger picture - pride in a place, a sense of community, and a general drawing-together of efforts. And no matter how desperate the need for support for a helicopter ambulance service or a hospice, a proper sense of community precedes all. London, or Sydney, or Budapest, or Oban all need to be nice places for people to live in order for businesses and services to prosper there. I think a few fireworks on New Year's Eve are part of that process.
I've just done a couple of minutes of googling, so I don't know how accurate these numbers are, but it seems as if the order of magnitudes are that there were around one million spectators in Sydney, and that the fireworks cost about AUD600,000. I think it is a reasonable bet that those spectators would almost all have been willing to give up at least 60 cents for the pleasure of watching the display, and that the average willingness to pay would have been much much higher.* So it sounds to me as if it was actually a very good use of resources, and not a "waste of money" at all. (This is separate from the question of whether the corporate sponsors get a return on investment; I'm simply asking whether the firework display was a worthwhile use of resources, and if the numbers I saw are right -- or even off by a factor of 10 -- I think the answer is clearly yes.)
* I'm using "willingness to pay" in a technical sense here: think of it as meaning that an individual is equally happy having $x and not seeing the fireworks, on not having $x and seeing the fireworks. Other boring economic discussion available on request.
also (and even though I am multiposting, can I point out that this is apparently only because everyone else is too hungover to speak) I want to make it clear that my comments are not intended as criticism of Dujon or Phil. Dujon's comment, and I think Phil's as well, were only addressing the question of corporate sponsorship of public events. I did pick up a more general sense that they were questioning whether firework displays themselves were worthwhile, but that may be my misreading, because it is certainly not explicit in their comments.