I've dappled in Spoken Word Poetry, and although I've never been to an official slam, I have to say I love it! I think the most important aspect is the voice it gives in a community that will listen without immediate judgement.
Often I feel like traditional writer publish their poems without reading them aloud. So when I read them aloud in my head, they are disjointed and stale. Spoken Word focuses on the flow and rhythm of the poem, which I think is an important lesson to any writer. (I'm not familiar with the distinction between urban word and slam poetry, but I assume it's along the same lines as the difference between slam and spoken word?)
Of course there is your good and your bad slam poetry, but on the whole, getting anyone writing/performing is a benefit for me! It not only helps self-expression, but interpersonal communication on the whole.