I disagree. What if you have one unit inside the cellmap of a building? Who do you click then? As well, the circle around the unit is 2D, while the unit itself is 3D. To further complicate overlapping, what about a unit that is slightly larger than the tile they stand on (say, they have a long sword that extends a bit). If that extends into a tile of another unit, then who do we select when clicking that sword? Even more, what if a unit is behind the building, and they're visible through the gap in the building. Does clicking through that gap select the building or the unit behind it?
Binding the selection to the model allows accuracy and ensures the player selects what they clicked on: every time.
If it's a big problem in specific models, though, you can always add an invisible box in the gap, which would allow it to be selected, although it would open those gaps up to the limitations listed above.
