The answer is brittleness. Milady's hair-replacement maintenance section states reconditioning treatments should be given as often as necessary to prevent dryness or brittleness of the hair.
The book also notes a second, separate benefit: reconditioning treatments can be used to liven up hair solutions that look dull and lifeless. Fading, matting, and yellowing are all real maintenance concerns for hair systems, but the specific purpose Milady assigns to reconditioning treatments is preventing dryness and brittleness.
Milady Standard Barbering, 6th ed., Ch. 15 (Men's Hair Replacement), p. 506: "Reconditioning treatments should be given as often as necessary to prevent dryness or brittleness of the hair."
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