As a general rule, the larger range a lens has, the more optical flaws it will have.
Also, you don't have to cover every focal length. Right now I have an 11-16, 50mm, and 70-200, and I have "full coverage" (although a 28mm is handy sometimes).
Plus, the Tamron doesn't have a constant aperture, so I definitely wouldn't recommend it. For $500 you can do better.