The StoFen is not a waste. It does have its uses. As digital paradise pointed out, its purpose is not what most people imagine/want it to be: soft, dreamy quality of light —it's not a softbox!—; its real purpose is to turn a directional light source (the fresnel lens of a flashgun) into a bare bulb (360° spread). It does suck up some light: ~2/3 of a stop, which isn't a humongous a loss as its haters claim it is. For indoors, events and the like it's useful: flash head turned 45° and aimed backwards (yes, backwards) with the StoFen on provides very good illumination. Outdoors it's a different matter of course, but then again, most slowlites sux outdoors anyway.
To be honest, the manufacturers of these gadgets are quite a bit to blame for the misconceptions: I've seen their sites where they go on and on about how much softer the light is with these things. Yes they do soften the light a bit, but that's not their main function.
At 10-15 bucks a piece, these are hardly a waste of money. People spend more money on things that are completely superfluous and several times as expensive as a StoFen.