I'm looking to get a diffuser for my Canon 580EX. However, the Sto-fen looks like somehting that the 580EX DOESN'T need. For instance, all it does is make the light pattern bigger and thus boucnes off more surfaces, which in turn makes the light softer and more even becsaue it is bigger. It also takes off about 2 stops, so the flash has to work harder and runs batteries down faster. Thus, it's useless outside. Also, the 580 has a pull out panel that increases flash to 17mm. And the 580 can swivels to bounce light also.
So my question is this: How would the sto-fen improve the quality of light on a 580? I'm thinking rubberbanding a white index card to the 580 in vertical mode would produce equal results and not cost more battery power?