I'll join the discussion here. :)

For my part, I don't think Arwen was weak. In terms of potentially ranking among Elvenkind, she was positioned to be reckoned one of their greatest women. She was a princess or lady from the noblest families in Middle-earth's Elvendom, raised and trained in the greastest center of lore and learning in the Third Age, and descended of Finwe, Elwe, and Melian. She had a lot of factors in her favor for turning out to be something other than a quiet, mousy little Elf-maiden.

Also, although Elrond had Arwen brought back to Rivendell because things were growing dangerous in the east, one must wonder if he wouldn't have ensured she'd be as well prepared to defend herself after Celebrian's capture and suffering as he could induce her to be.

Fighting was intrinisically a non-womanish trait among the Elves, but that doesn't mean they were incapble of acquiring the skills and devoting themselves to warfare. They were the nurturers and the men were the warriors, except as occasionally evidenced by people like Elrond (who apparently gave up warfare in the Third Age to become a great healer).

As for who else in Rivendell could have gone out against the Nazgul, I would venture to say there were undoubtedly still several Noldorin lords in the area similar to Gildor, leaders of groups of Elves who were living with or close to Elrond's community. The Elves named at the Council or in other areas of the book need not be considered the only lords in Rivendell and the adjoining lands. Erestor is presented as the chief of Elrond's councillors -- apparently there were more, though unnamed.

Perhaps Erestor himself was one of those Elrond sent out to look for Aragorn and the Hobbits.

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Parma Endorion: Essays on Middle-earth, Revised Edition