Automatic crawlers can be easily fooled by spammers using random-text doorways and other surfer-hostile abominations; furthermore, once a spammer has found the trick of fooling the automatic spider, he and thousands of other spammers can exploit the trick for millions of pages -- thus totally destroying the value of the resource for surfers. To see this in practice, try looking at Google and the ODP for, say, the website of a Las Vegas hotel.
Google has its own set of advantages -- as you say, it is quicker to pick up new spam, and in that process it even occasionally picks up a site worth visiting -- and that more quickly than the ODP could have.
Surfers are best served by a variety of techniques, from which they can choose whichever they think will work best on their current interest. The ODP is one of those varieties, it often works better than anything else available, and there is absolutely nothing comparable to it within its niche.
That is the best justification for continuing as it is. But if you find some other resource more effective for some current search of yours -- then use it! we won't be offended.
