Products is a comma-separated list of products, so you can have several products with the same event.  Adam was referring to applying the product variable functionality, meaning the ability to list values, but have it persist like a regular eVar (product expires on page view, so it doesn't persist).

You can also use a custom event as a "visit" metric.  Simply set the event on the first page of a visit.  This custom visit metric will now be available for all conversion reports.  Additionally, you can set one event on the first page of a visit, and a second event on the second page of the visit. ...

I agree that this should be a metric available to campaigns or any other conversion report out-of-the-box, but there is a work-around.  eVars, including campaign, can be related to events.  Events can be used to either count instances, increment by a numeric value, or increment by a currency value. ...