And not to mention, floating point numbers are already roughly logarithmically distributed. Logarithmic distributions are most important in large differing orders of magnitude, so having the piecewise-linear approximation of logarithm is good enough for proximity buffers.
Indeed, logarithmic depth is pretty much useless on modern hardware, but it wasn’t always the case.
On Windows, the support for DXGI_FORMAT_D32_FLOAT is required on feature level 10.0 and newer, but missing (not even optional) on feature level 9.3 and older. Before Windows Vista and Direct3D 10.0 GPUs, people used depth formats like D16_UNORM or D24_UNORM_S8_UINT i.e. 16-24 bits integers. Logarithmic Z made a lot of sense with these integer depth formats.
