hmmm currently i dun have much lens with me =) in my camera bag i intend to slot in 1 tamron 28-75, 1 UWA lens, a flashgun and camera body... thats all for my currebt setup....
12-24 will be a better setup for you.
Better FL coverage and wider FOV for large expanses and interiors.
Since you already have a 28-75, another zoom in the bag doesn't hurt

The DA15 is real nice and small, but its not as wide and imho, its better for a prime shooter who can sacrifice some FL versatility for some prime lenses mix-matching.