Zoom lens vs wide angle adapter?

I'm interested in buying the Tamron 18-270mm lens for my Nikon, as well as Wide Angle & Macro Lens Adapter (http://photojojo.com/store/awesomeness/wide-angle-macro-lens/

My question is, does it make sense to buy both or should I just stick with the Tamron?
I've heard that it contains a wide angle already because of the large zoom range, so does the Tamron already provide a wide angle view at 18mm?

Firstly what do you shoot most?
For me I would buy a 18-35mm and a 80-200mm
Get the best quality lenses you can afford. Do not get a 18-270mm the quality is not good. Its almost impossible to get good image quality from a lense which cover such a huge range in focal lengths.
Also macro adaptors usually worsen the image quality. Pick a good wide angle zoom and a good telephoto zoom you won't regret it…

www.photographyreview.com {some reviews on lenses}