@dlowan,
Potential benefits I can envision from regular testing:
1) As mentioned above, you can monitor values to predict when the kidneys are getting worse and euthanize before Miranda starts to
show you that she's feeling worse.
2) You can monitor her for progressive anemia. Cats in renal failure often stop producing enough erythropoietin (EPO) to maintain their red blood cell cells, and become anemic.
3) You can monitor her phosphorus levels, which can rise with kidney disease and make her feel quite sick.
So what to do with this information?
1) I suspect that you're attentive enough to Miranda to know when her overall condition is getting worse and probably don't need the blood work to tell you this. Plus, daily fluctuations may make testing on a given day give an artificially dire or rosy picture.
2) The effects of chronic anemia can be very difficult for an owner to recognize, because onset is so slow and because the body is very good at adapting to anemia that develops over time. Still, anemia can cause weakness, fatigue, malaise, and hypertension, and may be correctable with administration of EPO.
3) High levels of phosphorus really do cause a great deal of suffering, and aluminum hydroxide can be given to bind circulating phosphorus and make the patient feel better. If phosphorus levels are getting high, though, you're probably going to recognize that Miranda's feeling worse and you can have her worked up at that time to see what's going on and whether it can be fixed or not.
So, long story short, in your particular situation the main benefit I see from regular testing is to recognize developing anemia -- which is only useful if you plan to treat it. That said, if you're going to track her red blood cells, you might as well also be taking a look at other relevant stuff -- creatinine, BUN, phosphorus, electrolytes, albumin -- that can be got from the same sample.
All that comes with the disclaimer that I don't actually treat chronic kidney disease with anything other than the permanent solution in my current line of work...