For those who do not know, you can type "site:yourdomian.com" in the google search box to see which pages are indexed in google. When you go to the last page of the results, many will see a huge reduction of pages from the number they saw on the first page and you will see a message:
"In order to show you the most relevant results, we have omitted some entries very similar to the X already displayed.
If you like, you can repeat the search with the omitted results included."

My question is this, so what? These pages are still in the index, they are still showing up in ranking reports (some for very high volume terms) and still pass PR. So what is the drawback, if any, from being placed in these "omitted results?"

You can change the tags/copy of some of these similar pages to focus on different keywords to get it out of the omitted results, but is that the only reason or is there more to this?

Could just be bandwidth saving...

You can change the tags/copy of some of these similar pages to focus on different keywords to get it out of the omitted results, but is that the only reason or is there more to this?

I think you answered yourself in the lead to your question: More unique pages possibly leads to more exposure in the SEs.