If descriptions are set individually then Google will use them see Google Support
“We frequently prefer to display meta descriptions of pages (when available) because it gives users a clear idea of the URL’s content”
A further explanation of whats and hows: Google Selectively Ignoring Meta Description Tags?
From Rand at SEOMOZ. I agree – keep it natural…
We’ve talked about how search engines index, examine and measure text – I can assure you that the metric of density is absolutely, 100% unused by any of the major search engines.
And this from way back in 2005
The notion of keyword density values predates all commercial [...]
There are “over 200 SEO factors” that Google uses to rank pages in the Google search results SERPs. What are the search engine optimization rules?
Here is the speculation – educated guesses by SEO webmasters on top webmaster forums. Should you wish to achieve a high ranking, the various confirmed and suspected Google Search Engine Optimization [...]