rlange - 2:53 pm on Mar 30, 2012 (gmt 0)
1. I started a wordpress blog a while ago. It only has title tags, no description tags. It ranks like crazy for certain search terms.
2. I have some old web pages where the title tag and the description tag are exactly the same. I didn't worry too much about seo then and simply copied and pasted to save time.
For those pages Google ignores the description tag and replaces it by a snippet from the web page text. These pages rank very well too.
As I see it, descriptions are more about CTR than ranking, so I wouldn't even consider where a particular page currently ranks in your decision.
Whatever you do, don't write crappy descriptions because certain people insist that you should have a meta description on every page. If you can write good, concise descriptions, do that. If you feel that Google's auto-selected snippets are better than anything you could write, leave it alone. Google might save you from yourself by replacing a crappy meta description with an auto-selected snippet, but if you're not confident, don't risk it.