Well.. I don't know if central piwiki system for external wikis is legal in all jurisdicitons, especially in EU as it means transfering personal data to third party abroad. At least - pages using such external piwiki instalation should warn their user that their IP, date of access to webesite and browers user-strng is about to be recorded by third party... It also apply to the website which decided to use google analytics or any other such a system not installed on local server.