I agree. I think that each Eliza is important in her own way.

The first Eliza gives us a glimpse into the Colonel's past - his true depth of feeling and character. Through this storyline we are shown that the Colonel was, perhaps, once much more like Marianne than he now appears. While before we respected him we can now sympathise with him and hope for his future happiness.

I think the first Eliza is also very important because of what she shows us about Marianne herself. The Colonel tells us that Eliza and Marianne are very similar in temperament and outlook. This, then, shows us what Marianne might become if a different life course were to be hers. That is, Marianne exhibits similar tendencies to Eliza, and if Marianne were put in a similar situation to Eliza without having her character and her sense grow, she too might fall. Thus, I think Eliza acts as something of a foil to the glorification of romance that Marianne seems to be espousing in the beginning. I don't think I'm expressing myself as well as I ought, but hopefully you will understand my meaning.