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Meatpounder, highlighting doesn't mean parsing.

You can read John Scott's take on it, or run a few searches on pages of your own domain where keywords only appear in your URL and nowhere else on the page.

Say you have a page named /blow-jobs/freakygothicbiatchessex.html. You never mention the word "freaky" or "sex" on the page text or have any links pointing to the page with those words in their anchor text. Now, run these searches: "site:domain.com/blow-jobs/ freaky" and ""site:domain.com/blow-jobs/ sex". Assuming that's the only page you have under /blow-jobs/, Google will return 0 results in both cases.

Here's Vanessa Fox's take on underscores, and you can see the same reasoning behind spaces and why Google doesn't parse them:

And speaking of putting a dash in URLs, hyphens are often better than underscores. african-elephants.html is seen as two words: “African” and “elephants”. african_elephants is seen as one word: african_elephant. It’s doubtful many people will be searching for that.

Notice we also see keywords separated by underscores highlighted in Google SERPs.