The Norton Critical edition has an essay in it which suggests, among other things, that the whole sad story about Colonel Brandon's lost love Eliza may be redundant, and that one sad and ruined Eliza would suffice to make the point that Willoughby was unworthy of Marianne.
I want to write more about this, but first I thought I would ask for your thoughts. The first story makes us more sympathetic to the poor colonel and perhaps the romance of it (the elopement, the lost love) makes him more like Marianne in temperament than we might otherwise have thought.
But, what do you think? How necessary to the story is it? Do you think that one Eliza is enough?