3 months is a long overseas assignment in most roles, I guess your company would have to pay you enough to make it worthwhile. That's not the city struggling for housing stocks problem.
Military deployment, seems reasonable, I wonder how many deployable military are living in these houses; also wouldn't the location remain your primary residence and so avoid the tax?
Wanting to live somewhere else? That's exactly what the tax is for - pay up or change your residence.
You asked for reasons why someone might choose to move out of their house for 6 months or longer rather than rent it out. I gave a few; nothing more. By all means, I support the local government's right to pass whatever laws/policies/incentives they want as representatives of the residents and a policy could be good on balance even if it screws over a few edge cases.
BTW, most of our overseas assignments are 2 years initially. 3 months seems more like a long business trip, not an overseas assignment to me.
Military deployment, seems reasonable, I wonder how many deployable military are living in these houses; also wouldn't the location remain your primary residence and so avoid the tax?
Wanting to live somewhere else? That's exactly what the tax is for - pay up or change your residence.