I think that the problem is that, if you have characters that can't carry their own movie, you have to make time in the Avengers movie for their origin story. It's not just to show how they got their powers; obviously, you could just have Hawkeye walk in and say "I'm the best archer in the world" and leave it at that. It also creates interest in the characters. Remember, the only successful superhero team movie (at least if you're talking about theatrical distribution success) is X-Men and the sequels, and in that, the individuals all had the same origin, and the team's origin was simplicity itself. The obvious exception was Wolverine, who had his origin spread over three movies.
Also, WRT a couple of other points: did the original poster miss the point about "In terms of the additional I think Black Widow, sure"? They're making a distinction between the core Avengers--which have (after the very early history of the comic) always been Cap, IM and Thor, more or less--and additional characters. Which means that they might add in others, as well; has anyone else wondered why Quicksilver and Scarlet Witch never showed up in X-Men? Also, it's not clear whether they're saying that Fury will be in The Avengers or whether he'll just be in The Avengers.