I used to wonder these same things. Now I'm pretty sure that a blowoff tube just keeps your airlock from blowing off and opening your beer to infection. It might have some effect on the taste, but only really vigorous ferments will blow out anyway. Most beers the krausen just falls back in no matter what you put on top of the carboy. Having said that, blowoffs are a good insurance policy against beer fountains.
Up next: Big Brew Off competition between me and Kaptain Karma as one team, and my two roommates as another--We'll be brewing Pale Ales with specifications on malts, hops, and total yeild to see who's version is better (and to end up with ten total gallons of great beer).
Also up soon: Belgian Dubbel
Primary: Grampa's Woodshed Apple Smoked Porter
Secondary: Zombiefoot California Common, Chocolate Strong Porter
Drinking: Seamus O'Drunkagan Irish Red, Humble Pie Imperial Stout, Capricorn IPA