Garrisons, then, become completely useless, as unlike sandbags (which protect you better), there are often not enough windows for your entire squad to fire from.
So in this instance garrisons do, in fact, have a mitigating factor to their supposed strength.
I agree that entering a building should have the bulk of the delay, not leaving it, should be tested as an alternative.
Soldiers would be taking time clearing the building to occupy, but not waste time clearing out of it. Makes more thematic sense and is a little more intuitive.