Unique, relevant META descriptions improve SERP CTR.
One tactic is to use action words and include your keywords like people do when writing PPC ads. The key is to make people want to click through. Stuffing it with multiple keywords is unproductive.
For example, searching for "car insurance", you'll see geico's description:
Quote:
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GEICO auto insurance, online car insurance quote, motorcycle insurance quote, atv insurance, homeowner's, condo, and renter's insurance
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Too spammy.
As a counter example, for "bank loan", AdWords ad promoting SuperPages says:
Quote:
Need a Loan? Apply for Secured,
Unsecured & Bad credit Loans Today.
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You got the keyword "loan" in there to make the listing look more relevant, the text isn't a spammy list of keywords, plus you got an action word "apply" to drive higher CTR.
BTW, similar/identical META description tripping a duplicate content flag hasn't been proved. Similar META description will not cause pages to get de-indexed or be chucked into the supplemental index. They may contribute to or subtract slightly from your overall site profile but I've seen too many large scale sites with identical meta descriptions to believe Google will act on that signal alone.