[HairOfTheDog]

It would make little difference if the others stood or fell if the Ring was captured in Rivendell. Rivendell was the one under threat from Saruman...

This is a Straw-man argument. Gondor and Rohan had not been declined assistance.

[Overtaxed]

Rivendell was the one under threat from Saruman...

And Rohan wasn't? So Saruman sent that weasel Wormtongue to Theoden, closed the Gap of Rohan, and did battle at the Isen and Helm's Deep just for fun?