[Mithalwen]

It says in LOTR that Rivendell had not the strength to withstand the full might of Sauron. It would hold out longest perhaps because of the power that dwellt there and its geographic seclusion, but if all else fell it would fall. The ring quest was hope against hope and only succeeded because of the combination of circumstances.

It was the habit of the sons of Elrond to work with their kin. It would have been a different matter to send Rivendell's general - it was Glorfindel who led the forces of imladris against the witch king. By the time of the War of the Ring, Elrond is a healer not a warrior - and the elves believed that the two roles were incompatible. Glorfindel was probably believed to be vital to the protection and in worst case scenario, evacuation of Rivendell.