I'm guessing dustbowl was built in a spiral because of goldsrc limitations of the original.
Goldrush is the same, because it basically takes the basic concept of dustbowl and applies it to a new game-type.
Badwater spirals so that RED can have a forward spawn without actually needing a forward spawn room. It helps with spawn camping, and helping players learn the final part of the map before the fighting starts.
You'll notice that Thunder Mountain does not spiral around itself, nor does Gorge. Mountain Lab does it a little differently, and Upward is done for it's mountain peak theme.
Hoodoo and Egypt are not spirals as well, and they play perfectly fine (actually pretty popular, especially Egypt)
Everything said, do whatever suits you the most. Just make sure your map is optimized well enough. Trying to copy other maps too much will make yours feel like yet another clone.