As some others have answered, it helps a little...but it's not one of the primary factors.
One thing to consider is whether or not including keywords in your URLs makes them considerably lengthier. As SearchEngineLand points out in their 2012 Guide to SEO,
...various research reports over the years have shown that searchers are more likely to select pages with short, descriptive URLs over other pages in search results.
So if you can make your pages have descriptive, keyword-inclusive URLs without making them overly long, it may help - but won't hurt - SEO...and it can possibly help with your clickthrough ratio as well.
(As for just how much it'll help? That's up for debate.)