The advantage of themes over DW templates is simple:
1) If you update your theme you don't have to update all of the pages that are derived from it. If you update a DW template and have 1000 pages on your site you would have to update all 1000 pages, with themes you just update the one theme page.
2) With DW templates you can either have the php code above the <html> come from the template and be locked on the page, or come from the page and be ignored on the template. Themes allows you to add php code to the theme and to the pages and they both work independently.
3) You can actually have more than one theme for a given piece of content. If you are using a DW template, that is your template and to change your template you need to update the page. The theme is fully dynamic, so you could actually have the equivalent of multiple different templates for any given page that could be personalized by the end user, admin, or developer without having to update the code.