What is a "black hole" website

Some sites appear to be biased by never showing links to other websites, or if they have external links, they are all nofollow. Such sites are called "black holes", a metaphor referring to cosmic objects whose attraction prevents anything to leave.

The best known example of the first case, the New York Times shows links on its own articles only. The latter is exemplified by Wikipedia, which offers a wealth of links, but all nofollow.
We will discuss the interest in terms of optimization of this practice.

1) Is the effect positive?

Based on the traffic of the sites in question, one might think that this is not harmful, but we must look more closely. The NYT has a great reputation and is considered a reliable source of information, it theoretically does not need to cite sources, except when talking about another site ... However this practice hampers surfing on Internet.
To access their pages a password is often required, crawlers can not scan them and so, SEO is not a priority to them.

The case of Wikipedia is also different because with its TrustRank (it is favored by engines) and FreshRank (constantly updated content), it does not rely at all on PageRank.
Using nofollow is perhaps required on collaborative sites to prevent spam, it is not the case on other sites.

2) Can us recommend that?

The Google algorithm is almost entirely based on the links to determine the relevance and value of a page to a query. This has made its success and allowed it to remove the Altavista search engine from the landscape. Without the links, the Google engine does not worth more than Altavista.
If actually there is no penalty in ranking, especially on trusted sites, it is expected that this change in the future.

3) Does it give an advantage?

The advantage of not putting links on a page to another site exists only in the case or the page on the other site links on your own page. Cross-links, especially if they are frequent, have no value.

In all other cases external links enrich the contents of a page and promote its selection for the results, and selection is more important than ranking, content is more imporant than optimization.
Knowing that we can see sometimes on top of results pages made primarily of lists of links, one can doubt of the value of the black hole policy. For example Digg.com which depends only external links on the pages of contributors...

And apart from the case of Wikipedia to which beginners proudly put links on each occasion, we should not expect any support from webmasters when they see a site that applies such a policy.
And Net surfers appreciate exploring the Web, so a site with no external links is of lesser value.

• SeoQuake. This Firefox extension has an option to mark nofollow links in visited pages.
• NYT opens its content. I don't know if they have read my article ;), but they have now choosen to open their content to third parties (Dec 4, 2008). A proof that black hole site was not good.