Undoubtedly, old, used comb has many advantages. Among them, their strength and saving the bees the effort of having to build new combs when their efforts could instead be used for storing honey. After all, in order to produce new wax, the bees have to consume honey. I love extracting honey from old, strong combs that hold their shape when spun in the extractor. That having been said, there ARE reasons to cycle old combss out of use.
Already mentioned are the increased possibility of hosting disease-causing bacteria and containinig residues from poison applied to the hives (like for the control of varroa). Not yet mentioned are the fact that, as combs are used for more cycles of brood rearing, they become smaller, as the exuviae of the developing bees are pushed againt the insides of the cells. At some point, the cells become too small for the development of full-sized bees. Another problem with old combs is that their color leaches out into the honey stored in them. If you like a dark honey, no problem, But if you like your honey light colored, you are more likely to get if if it has been extracted from new. light combs.
As mentioned by others, mostly, I cycle out damaged combs with broken areas or with too much space dedicated to drone cells.