In order to get listed, you have to "push" change notifications to the service, since it is not feasable for them to check all the registered sites for changes at useful intervals. The "push" consists of a "ping" -- a HTTP request containing all the info weblogs.com needs in order to update the list.
Sometimes these trackers also poll each other. blog.gs, for example, says that lists of updated weblogs are imported from antville.org, blogger.com, pitas.com, and weblogs.com are imported every hour.
If there are too many blogs available, finding useful blogs is getting more important. "Last updated" is not a useful category for most people, specially when there are a gazillion updates per minute. The really important tools analyze the activity in the BlogoSphere. Since these tools need to get a list of blogs to analyze from somewhere, weblog trackers still have a role to play -- they feed data to other tools. Without trackers, they might have to rely on manual submissions.
TechnoRati , for example, tries to identify hot topics and lists some blogs referring to them. You can be sure Technorati wouldn't list 300,000+ blogs without trackers such as weblogs.com.
Other interesting experiments like http://www.brainoff.com/geoblog/ show the world as a blog. It's a Flash applet which reads the changes.xml from weblogs.com, looks for blogs with GeoUrl metadata, and then plots the postings in real time onto a world map.
The problem for wikis is that wikis don't usually have a front page like a WebLog. RecentChanges has all the postings and summaries, but does not link to other sites. The various pages do link to other sites, but then you would have to treat each page on your wiki as a single weblog in order to take advantage of blog-tracking services.
Solution: Pass every page as a new site to these services.
Another problem is that these tools often do not accept URLs with parameters.
See also ChangeAggregator.