While it may be the case that this version of their homepage has been tested to yield the best conversion, that seems completely irrelevant to my point: "the Basecamp homepage does a very poor job of communicating what it is".
You find it hard to believe that people would pay for things without understanding what they are? I don't understand your premise.
Do you honestly think the Basecamp homepage communicates what it is? As stated earlier, the description could apply just as well to Dropbox, MediaWiki, or Eventbrite.