Is the "boon to the local economy" mantra exaggerated?

Every time there's a queen or a pope visiting or some other event or happening taking place in some part of the country you hear about how the locals will allegedly never see another poor day but how good are these one-off events really for the local economy?

I went to some island not too awful long ago where a festival had been held. Lots of outsiders were drafted in for the running of the festival and the festival-goers brought in as much supermarket bought food and drink as they could carry and left behind a big mess..needles and all. The ferryman could probably afford to go on the lash for a few nights after but that was about the size of it.

If the place doesn't have much of a local economy to start off with it's even more grim. The owner of the local Spar will get some more money to give to BWG Foods in Dublin and buy some shiny new self checkouts to make a few locals redundant. Or maybe the owner of the local coffee shop will have a few extra bob to go on a sun holiday in Aruba but little more than that.