I've used one blow off tube, I believe. I think it was about three years ago.
I use a 7.5 gallon bucket for primary (almost always), so I have plenty of headspace for a 5.25 gallon batch. I also always ferment on the cool side of the yeast strain's optimum temperature, so I rarely get super vigorous fermentations. Even using a pacman starter last week, my IIPA (OG 1.072) is fermenting at 60 degrees and there isn't any krausen peeking out.
I know that a warmer fermentation can have explosive results, as can using a big percentage of wheat. I also think that some people don't use a large enough fermenter to give them enough headspace. If any of those things are happening, then a blow-off tube is probably a good idea!