I cannot comment on your user test results since I do not know your parameters and scenario.
But, talking about gmail's new email input method. The advantage which desktop email applications had over the web based ones was, while composing the email (in a separate window) you could freely browse older emails and look into content you might want to refer. (This was possible in web mail too by using different windows, but was not quite clean). With the new docked email method, you can keep typing your message and browse other content too.
Also, this is quite similar to what facebook does. If you look at the wording, it says 'Message' and not email. Slowly and gradually, mail is being merged with chat. The storage mechanism is same for chat and mail as is. In facebook, a message and mail are the same thing. When you use chat or mail client to contact the person, they end up in the messages inbox.
To answer the question as to why this is not a popular UI solution, you need to ask, in what scenarios is this a good solution? The answer is something like multitasking: focusing on a task while managing others in the same view. How many services do you think need such a feature? Many services already use pop-up windows to achieve similar results.
If used unwisely, this will amount to quite a lot of cognitive overhead for the user. On a streamlined app, it makes sense (like gmail).