First there were separate browser windows for each page, then there were tabs inside a single window, and now we have tabs within tabs. I believe this is one grouping too many and personally I'm fine with just dragging tabs between Chrome windows.
Also the UI (at least Opera's) makes it very clear which tabs are actually tab stacks, and presents it in an intuitive looking way. People who have never heard of the concept and then use my browser typically grok it almost immediately just from the design.
I agree, we're iterating this concept too far for my UI tastes.