I've given this some thought, apologies for the long post incoming!
In theory a multiscale should be useful for mitigating the more extreme differences in gauge between strings for a balanced tension set. According to
Mersenne's laws, string gauge and note frequency are inversely proportional when all other variables are fixed (string tension, string material, scale length). When you go up in fourths, that's a reduction in gauge by 3/4 between each consecutive string, but in fifths the reduction is 2/3. This quickly adds up with the larger gap in fifths tuning: a 5 string with a .130" diameter lowest string in fourths would need a highest string with .040" diameter, but for a 5 string in fifths the highest string would need a .025" diameter.
In practice the effect might not be as pronounced. For a 5 string tuned CGDAE (like mine!), and aiming for 40 lbs of tension spread out evenly across the strings (in the least squares sense), I used a regular 34" scale against Ibanez's
EHB1005MS specs (assuming a uniform .5" decrease in scale length) with Kalium's string tension calculator. The differences are minimal. But if I use the
BTB605MS specs, then we see a lot more deviation in the lower strings. You can see the screenshots in the attached images.
To me the real benefit of a multiscale is for tuning
lower to avoid the
inharmonicity of thicker gauges. Typically fifths tuning mirrors the open strings of the violin and mandolin families, so C1 is the lowest most of us go for. This isn't a strict requirement, and you can tune as low as you want. Sometimes I dream of having a 37" multiscale for a low F0...