Below is a Grade 1 Evidence Based clinical trial. CRF cats that ate a
protein and phosphorus restricted food lived more than twice as long
as those that did not. Not contained within the abstract below was an
additional important piece. Cats fed the renal failure diet had less
than half the number of uremic crisis and the time between uremic
crisis was greatly longer. There's a huge difference between peer
reviewed published grade 1 evidence based medicine and the latest
internet spawned hypothesis. Nobody in thier right mind would suggest
phosphorus is not acutely a part of the issue with CRF kitties.

J Small Anim Pract. 2000 Jun;41(6):235-42. Related Articles, Links  
Survival of cats with naturally occurring chronic renal failure:
effect of dietary management.

Elliott J, Rawlings JM, Markwell PJ, Barber PJ.
Royal Veterinary College, London.

Fifty cats with naturally occurring stable chronic renal failure (CRF)
were entered into a prospective study on the effect of feeding a
veterinary diet restricted in phosphorus and protein with or without
an intestinal phosphate binding agent on their survival from initial
diagnosis. Twenty-nine cats accepted the veterinary diet, whereas
compliance (due to limited intake by the cats or owner resistance to
diet change) was not achieved in the remaining 21. At diagnosis, both
groups of cats were matched in terms of age, bodyweight, plasma
creatinine, phosphate, potassium and parathyroid hormone (PTH)
concentrations, packed cell volume and urine specific gravity. Feeding
the veterinary diet was associated with a reduction in plasma
phosphate and urea concentrations and prevented the increase in plasma
PTH concentrations seen in cats not receiving the diet. Cats fed the
veterinary diet survived for longer when compared with those that were
not (median survival times of 633 versus 264 days). These data suggest
that feeding a diet specifically formulated to meet the needs of cats
with CRF, together with phosphate binding drugs if required, controls
hyperphosphataemia and secondary renal hyperparathyroidism, and is
associated with an increased survival time.

Paul,
   I am assuming you have a CRF cat because the only indication
(application) that Royal Canin gives for the diet is for renal
failure. It is certainly possible that your vet has detected some
other issue and has decided the nutrient values in the LP pouch meet
the needs of some other disease. It may be s/he is considering liver
disease and renal failure product has similar nutrient profiles as
that of an hepatic diet. As always, your vet is the best person to get
accurate information from.