The question becomes what is the incline of diminishing returns for cables?
My own guess is that it's pretty steep, because compared to other elements in the system, cables make a smaller contribution.
Thus, the killer question for anyone spending on cables is: Are cables more deserving of your upgrade dollar than anything else in the system?
For example, if one were to spend $4k on interconnects, the question I'd ask is whether $500 in interconnects and $3500 elsewhere in the system would be a better investment. That's just the form of the question, and everyone would fill in the amounts with whatever suits their situation.