For example you want to have a description of a city guard, and draw “Corruption: Power is misused, or someone is corrupted by power” from Story Forge Cards, that is bound to give you a good idea on how to play that character. While if you used Story Cubes and rolled a turtle, you’ll play him very differently. In absence of these more specialized products, you could use a standard Tarot deck for the same purpose. In fact it could be said that “Tarot reading” is very much the ability of inventing a story from random cards with powerful images.
The main advantage I see is overcoming stereotypes, where every innkeeper appears the same to the players, because you have some image of a typical innkeeper in your mind from films or books, and just play all of them like that. The disadvantage I see is that the products I mentioned aren’t necessarily made for fantasy roleplaying games; writing a novel is not quite the same as writing a pen & paper RPG adventure, and some of the cards or dice might not be appropriate for you story or setting. But then you can always draw again or reroll.
I’ve seen Story Cubes been recommended for DMs to come up with things on the fly. I’m not convinced, rolling dice in front of the players sends a strong message of randomness, and immediately tells them that this character isn’t central to the story. Maybe with the iPhone app hidden behind the DM screen, but I’d rather use the cards or dice for preparation, and just produce some extra characters or plot elements in case the players send the story in a direction I didn’t predict.
It appears as if the Story Forge Cards aren’t easily available in Europe, I’d have to import them. I think I’ll try out the cheap digital version of the Story Cubes first and see if they can help me write adventures. And I’m sure I still have a Tarot deck from a previous role-playing game somewhere.