There are two models that I would find plausible:

- Include providers by wide use and/or wide acceptance
  This would mean that those providers are shipped, that are used by most of our users or alternativly that are 
  most widely known. Of course there is a lot of special case in this, but I in this model I would ship Webdav 
  and google and provide caldav and wcap as an extension.
  Provider
            
- Include providers by coroprate size and functionality:
  The "smaller" providers, webdav/caldav, have a quick server installation for anyone on their (home-) server,
  which I think would be the criteria for deciding which providers are in. Google providers no downloadable server
  and I'm not sure how accessible WCAP is, but I have the feeling its very bound to corporate use, which would be
  the criterium for excluding those.
  
Philipp