I've only needed one a couple of times. Usually big beers and when I've put 5.5 gallons of wort into a 6 gallon carboy.
I imagine you'd want one if you are dry hopping in a primary too. You don't want a leaf plugging up the airlock hole.
With buckets, I just take the airlock grommet off, slide it around my siphon hose and fit the grommet back on the lid.
Planning: Agave Witbock, Raisin Beer
Primary: GF Hazelnut Stout
Tertiary: Cranberry-Pom pLambic (est. bottle date: 03/01/08)
Drinking: Cab.Sav/Merlot Wine, Grand Cru, Hazelnut Stout #3, Ordinary Bitter