Not rare per se but kind of rare on Gassers of the era from what I've seen/read.
Benefits are improved throttle response and low end torque over a larger carb. Only negative, IMO, is you can lose some horsepower but that's normally not a big deal as most of us can't use all the power that any modern enduro bike has.
As for jetting, it will tend to be smaller on the main than on a 38mm carb as the intake velocity is greater which in turn pulls more fuel up through it.
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