Howdy,
Fair criticism. Our cache documentation is probably not all it should be. We do have guides for Magento, IPBoard, and vBulletin, but not for WordPress.
Page caching is often not a simple cookie-cutter process. Different sites have different pages that can or cannot be cached and the page cache need to be set up specifically for these sites. We are currently working on ways to make this process easier, but it's very difficult to make something that will both effectively cache content that should be cached and not cache content that shouldn't be. You will notice that just about all page caching solutions have this learning curve.
That is the reason it is hard for our techs to answer questions like "Whats the htaccess code for enabling it for WordPress and other popular scripts at least?" It need to be customized to your site. You will be able to get much more out of our support if you try specific rules yourself and then ask us questions about why these rules did or did not work. In this sense, though, you are right — page caching can still be a difficult beast.
As for private vs. public cache for shared hosting, I would say "private cache." Private cache is not very efficient, but you at least won't serve content to the wrong people accidentally. Public cache is not something you want to enable for all users. In fact, page caching (any page caching) is not particularly compatible with shared hosting.
Michael