Gain generally isn't changed so often that a POT would be necessary. Plus, the feedback loop is a pretty critical part of the circuit, high spec metal-film resistors will have lower coloration/distortion/etc of the sound.
Plus, a GOOD pot is pretty expensive, you wouldn't want a piece of junk. Plus the're relatively pretty large (even moreso for a good pot, on average). Plus, one can match resistors or buy ultra-high tolerance parts but it's a much more expensive proposition to buy a ton of HQ POTs just to pick out two you'd use together.
IMO, if you feel you really need an adjustable gain, it would be better to use a good DP (dual pole) multi-position switch to switch between multiple different gain resistors-pairs.
Plus, often you already have two volume controls, the one on the amp and often the source. SImply chosing the higher of the two gains you'd ever need could suffice. Having to turn the volume knob in finer increments instead of a POT adjusted gain could be the lesser of two evils.
Also consider that Tangent has mentioned that original values are used in the guides but that most people don't need that high of a gain (except if there is crossfeed or other add-on circuitry to lower the signal strength, but that is generally a fixed/known situation that can be factored in when choosing a single gain setting).
If you only had the POT accessibe inside the case, then is there even much value in having it? If you have to pop the case open anyway you can just swap a socketed resistor(s).
