Many search engine marketers have historically recommended placing relevant keywords in URLs.  The original rationale was that a URL's contents are one of the major criteria in search engine ranking.  Yahoo especially was enamored of keyword-rich URLs.  

Over time, this has changed.  It is now a minor criterion with regard to ranking.  On top of that, dynamic sites make the process more difficult.

That does not mean, however, that creating URLs with relevant keywords is obsolete.  God no.  Thinking like that is terribly narrow-minded.  So let's enumerate all of the benefits of placing keywords in URLs.  Here they are -- my favorites are bolded:

1. It still probably still has a marginal effect on search engine ranking in and of itself.
2. It is roughly 20% of the real-estate you get in a SERP result.  It may function as a call to action and increase perceived relevance.
3. The URL appears in the status bar of a browser when the mouse hovers over anchor text that references it.  Again -- it may function as a call to action and increase perceived relevance.
4. Keyword-based URLs tend to be easier to remember than "?page_id=5&category_id=2."
5. Sometimes the URL is cited as the actual anchor text, i.e. <a xhref='http://www.example.com/foo.html'>http://www.example.com/foo.html</a>.
    a. A user would likely click the anchor text including relevant keywords over a dynamic string.  Same story here -- you know the effects.
    b. Since keywords in anchor text are a decisive ranking factor, having keywords in the URL-anchor-text may help you rank better for 'foos.'

I really do think that the current keyword-in-URL debate is missing the point.  Even if, as everyone suspects, reason 1 is debatable, the above enumerates 4 more really good reasons to consider URLs in a search engine marketing strategy.