Not long ago Google Blogger platform added new widget-driven templates, which use different approach from the "Classic" template. Classic template was based on HTML, new Blogspot templates now are based on XHTML which is a branch of XML.

So, classic templates are incompatible with new Blogger templates. New blogger templates use "Page Elements" (widgets) but the places where you can insert widgets are rather limited. So what about if you need more flexibility than they allow?..