Many people have several different domains and would like to know whether they should or not link them all together. In fact, most people have absolutely no use for so many domains, however some sites such as casinos, medical companies and also stores have these many domain names. It is advisable not to link them all together (even if they are about the same subject or even linked somehow) because it will certainly look quite spammy for the Google Webmaster’s staff.
However, there is a situation where people can, and should, link all of their several different domains together. This situation would be if you have several different domains that had exactly the same content. How can this be, you might be asking yourself. The answer is quite simple, although these domains are the same they are showed to the public in different languages. This is one example of a case that people should link the sites together, that way the public will have a much nicer chance to finding it on the language that they need.
Instead of placing all the full links to your web site’s main page you could provide it in another way, one that is much clearer and lighter in a way. For example, you could have small flags representing the language that the site is on. That way, whenever a person clicked in such flag he would automatically be taken to your other domain. The most important thing, especially for search engines, is the fact that these links should be brought to people the clearest way possible. The easier it is for the public to deal with your page the more likely you are to reach a better ranking with Google.
Make sure you do not stuff your pages with other domain names. This would compromise the user experience as well as make your Google ranking go down.
Video: If I have 20 domains, should I link them all together?
(YouTube Release: Jul 17, 2013)