Cats have a naturally low thirst drive and it's important for them to be fed wet food regularly to reduce the specific gravity (i.e. concentration) of their urine. It's better that you feed a cat with urinary issues wet food (any brand/variety, but preferably high in meat/protein content) than to continue feeding them prescription dry food.
Lisa A. Pierson, DVM, says specifically of cats who have been diagnosed with crystals in their urine
Always keep in mind that water flowing through the urinary tract system is the most important factor in keeping it healthy. That said, please do not make the mistake that so many people make when they state "but my cat drinks plenty of water!"
Unfortunately, many of these people and their veterinarians have
missed the point of water...water...water and have continued to put
the cat in danger by feeding/prescribing a dry food diet - including
any and all of the prescription dry diets.
It is highly counter-intuitive to label any water-depleted (read: DRY)
food as a "urinary tract diet."
Generally speaking, the basic diet recommendation for the average cat
with urinary tract issues is a high protein/low carbohydrate canned
food with added water.
Her web page is highly educational, and I recommend it.
Prescription diets are high in starch and fiber compared to a quality wet food diet with a high level of protein. This quality can make your cat more susceptible to struvite crystals.
Starch and fiber in diets potentially stimulate formation of struvite crystals. Hence, reducing dietary carbohydrate is desirable to prevent struvite urolith formation. In addition, a net loss of body calcium, phosphorus, and magnesium during feeding of the fiber diet suggests that dietary inclusion of insoluble fiber could increase macromineral requirements of cats.
Am J Vet Res. 2004 Feb;65(2):138-42.
Evaluation of effects of dietary carbohydrate on formation of struvite crystals in urine and macromineral balance in clinically normal cats.
Funaba M1, Uchiyama A, Takahashi K, Kaneko M, Yamamoto H, Namikawa K, Iriki T, Hatano Y, Abe M.
Prescription diets tend to have less protein than a quality wet food diet. This quality can make your cat more susceptible to struvite crystals.
Our results indicate that compared with dietary supplementation with NH4Cl, the high-protein diet is preferable as a urine acidifier for the prevention of struvite crystal formation in clinically normal cats.
Am J Vet Res. 2003 Aug;64(8):1059-64.
Effects of a high-protein diet versus dietary supplementation with ammonium chloride on struvite crystal formation in urine of clinically normal cats.
Funaba M1, Yamate T, Hashida Y, Maki K, Gotoh K, Kaneko M, Yamamoto H, Iriki T, Hatano Y, Abe M.
Acidification of urine can decrease the risk of struvite crystals, but can increase the risk of oxalate crystals.
Manipulation of urine pH through dietary means has proven an effective
tool for the management and prevention of struvite urolithiasis;
acidification of urine, however, may be a risk factor for calcium
oxalate urolithiasis, which now appears to occur with approximately
equal frequency in cats. Prediction of urine pH from dietary analysis
would thus be a valuable tool, but considerable further research is
required before this can be achieved with commercial canned foods.
With the growing importance of urolith types other than struvite,
alternatives to the measurement of urine pH are required to assess
critically the likely beneficial (or detrimental) effects of
manipulation of nutrient profile. ...Recent observations suggest that
recurrence rates of signs in cats classified as having idiopathic
lower urinary tract disease may be more than halved if affected
animals are maintained on high, rather than low moisture content diets
J. Nutr. December 1, 1998 vol. 128 no. 12 2753S-2757S The Effect of Diet on Lower Urinary Tract Diseases in Cats. Peter J. Markwell, C. Tony Buffington, and Brigitte H. E. Smith
Credit to Cat Centric for finding the scientific studies.