The obvious application of this, is to make a server which agregates change information from many wikis.
recentchanges.net is unfortunately now offline. This was an implementation of a change agregating server. This website was also the main resource describing the wikiping XML standard.
[Wikka wiki's WikiPing page] has some information. WikkaWiki was the only engine to support wikiping, but now it seems these wikka installations have nothing to ping to! So it looks like the idea is dying a death now, unless someone else hosts an aggregation server. Aparently the wikka folks still have the code for such a server.
See also ModWiki, our comparable 'pull' RSS standard. Although ostensibly, one should just push the RSS feed.