What is the benefits of Panels for a themer?
One large advantage of Panels is that you can have wildly different layouts for pages, depending on your needs.  In this context, layouts can mean both content and styling.

On the content end, you can have your main node-being-viewed piece, and add in different things.  For example, you can add in views into sidebar areas that show/link to related content.  You can also add in blocks, and have a lot more flexibility than what come stock with Drupal (though D7 really improves things).

On the styling end, the biggest advantage is being able to choose different layout templates for each page and variant.  Other modules, like Context can do similar things, but I find it much easier to do with Panels.

With the sites we build, all pages are Panel pages (no exceptions).  From the client's perspective, having adequate training and inline help greatly helps with them being able to keep the site alive (and nearly all of our clients are non-technical).

A side effect of using Panels is that you get a lot of features that cTools provides, which can be a huge benefit when you need to retrofit a site.  In a lot of ways, we have found using Panels pages for everything really aids with future proofing a site.