A linear relationship between output voltage and the resistance at output is the most desired relationship.

This is because when reading from an ADC, ideally one would want to read voltage values with mostly equal differences between combinations that give different equivalent resistances at the output.

For example, if the relationship between output voltage and output resistance were logarithmic, then it would be difficult to distinguish one combination from another due to both having different equivalent resistances at output that produce nearly the same output voltage.

If the relationship between output voltage and output resistance were exponential, adding a small resistance would produce a negligible effect on the output voltage.