Optimal string tension

[Winspear]

Especially with extended range, the inharmonicity is going to be so much larger on the thickest bass string that the others could be neglected.

Indeed. And whilst those thin strings will indeed be more..harmonic(?) being longer (take a listen to the pianolike trebles of extended range basses), when given the same bass length (eg 27 vs 27-25 fanned), the slight warming of the treble strings by the fan and/or perhaps thicker treble gauge is useful as you can dial the overall tone a little brighter to counteract dullness of the thickest string. In other words, inharmonicity is only really a problem for the thickest strings, and a fan helps balance the tone a little due to this