It might be more common that I think but I have never seen it anywhere personally.
The depot protect the ramp, the SCV, the wall of the main.
The bunker is really well protected by :
Cliff
Ramp
Mineral
2 depots
Zerglings can't really attack it with any efficiency. It's not so easy for roaches and even banelings can't hit it easilly. Plus there is still room behind for SCV to repair.
1 tank can be placed on the cliff and protect the whole defensive area without being exposed itself.
I found that depot placement very cost efficient especially early game when you can't wall quickly all your Natural ramp.
You surrender the natural ramp but when you think about it, it's not really the vital zone you want to protect, it's more the main ramp, the Natural and the depot itself. The Natural wall as we know is very susceptible to busts because it's very large.
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