I've given this a little more thought, and come to the conclusion that the attribute potential maximum system is working as intended. Its goal is to slow down natural training, and that is precisely what it does, by a factor of 2 or 3, when an attribute reaches its "maximum."
The problem is the relative ease / difficulty of acquiring natural training, compared to "magical" training; and also relatively among various attributes.
I think it's fair to call herbs a magical means of raising attributes, even though the technical means by which they do so is the training system. If herbs and potions had the same relative availability, then Herbalism and Gardening wouldn't be nearly as interesting. I like their existence, but I still agree with the original poster that it would make for better gameplay (and easily rationalizable) if herbs were not limited to some arbitrary absolute value, but rather respected a character's attribute potential maximums.
The real problem IMHO is this:
Consider the difference between training Mana, Strength, and Dexterity.
A spellcasting class, whose prime attribute is Mana, which helps determine their ability to attack monsters, needs only do one thing to continuously, naturally, train Mana up into the 40's and beyond: cast spells. This comes naturally during the course of attacking those monsters, and while doing all the other things a caster naturally does by playing the game.
A melee class, whose prime attribute is Strength, which helps determine their ability to attack monsters, cannot train Strength naturally beyond 18.
A ranged class, whose prime attribute is Dexterity, ... ..., cannot train Dexterity naturally at all.
In summary:
Caster: Mana; trains into 40's during natural course of play.
Melee: Strength: cannot train past 18.
Ranged: Dexterity: cannot train.
IMO it is nonsensical (broken, actually) for weight training to be limited to an absolute value of 18, when the statuses Burdened, Strained, Strained!, and Overburdened! are themselves relative to the strength of the character. Is it impossible in the real world for a strong man to exercise his muscles by lifting weights?
I would also suggest that firing missile weapons of any type ought to provide natural training for Dexterity.
The current implementation of attribute potential maximums works great for slowing down natural training that is provided by spellcasting, weight training, and all other means of natural training. But the availability of that natural training is grossly imbalanced, and in some cases simply missing.
P.S. I realize it's also possible that we, the players, are simply unaware of the intended means of training Strength and Dexterity, and that no one in all these years has stumbled upon a reliable means of training Toughness, etc.