When we use them, it's cause we really need them: and tip appropriately.

They have: gotten us on the next flight out of a log-jammed airport while 5000 other people stood in lines crying and clueless, stranded -$20 tip; and hailed us the next cab with no waiting while 250 people stood in line - $10 tip.

In both instances, the service provided was available to anyone for the asking, with tipping optional; but evidently nobody else wanted to part with the bucks, or knew what a skycap can do.