Nazgul: Reply to Dennis & Paul
John R.

I don't think the Nazgul were afraid Frodo would put on the ring and command them: Frodo was too weak and inexperienced to do so, a point made by Gandalf, I believe, somewhere along the way. The thought probably didn't even cross the Nazguls' minds: anyone mastering the Ring - and thus being mastered by it - would decide to keep and utilize the Ring's greatest servants, not destroy them, or the Ring.

I don't agree that the Witchking shrieked at the name of Elbereth: he certainly didn't do so at the Ford when Frodo said the name again. No, he shrieked because a Westernesse blade wielded by Frodo cut through his robe, and almost through him. Again, this likely led to a strategic retreat.

Finally, remember the detailed description of the Black Riders' journey set forth in the Book of Unfinished Tales (I think that's the one). Sauron had commanded them not to reveal themselves, so theirs was a stealthy mission. Sauron attacked Osgiliath solely to provide the Nazgul with cover so they could slip across the river undetected. Sauron wanted to conserve his resources, seeing only the military threat, so did not engage in a more aggressive effort to recover the Ring. Militarily he could destroy Gondor even without the Ring. Sauron was overly confident, at least until Aragorn shook him up with the Palantir later on.