Search engines offer a free service as a matter to promote non-free services. Otherwise, they won't survive and other companies will born to do it better (whatever it is that allows them to success, survive). So they need to be competitive and attrack as much users as possible to their non-free stuff... I guess, everyone will agree with that.
In the course to be competitive, they must learn how to index good stuff (regardless of the URI) for we all to easily and fast find using their service (otherwise we'll use another SE). In fact, it is like this, there are millions of pages indexed that use "non-human friendly" URIs.
And everyone use SE, because such a service is needed by everyone on earth. And guess what, many search engine companies do their job pretty well. Most of them rock! Everyone talks about all the goodies they do. ...as a side affect, all those that create content get worried about that. Every webmaster wants to be indexed, but poor boys, the preasure is now on your side, because you have to offer "good SEO stuff", otherwise the best page rank will be assigned to someone else. But what "good SEO stuff" is? So many have tried to answer this simple question (and many charge to help webmasters apply their magic SEO methods, oh well). The answer is pretty simple, it is "competitive content", because that's what people expect from search engines.
However, webmasters still demand "human-friendly" URLs, so that's what many products (bulletin boards, portals, cms, etc.) out there are giving them. But that's far, far from what search engines need.
...and that's it. No matter the URI, what matters to everyone is "content". Ask yourself what do you expect from a search engine.