I believe that one of the important tasks of a banner bearer was to protect the banner. If the banner was captured, it was considered a disgrace. Therefore Elrond as the banner bearer would have to have fought at the Last Alliance, both to protect the banner, and because he would have been a rather conspicuous target carrying the banner of the elven king.
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His sword was long his lance was keen
His shining helm afar was seen
The countless stars of heavens field
Were mirrored in his silver shield
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