Questions about "prescription dog food"?

My siberian husky has crystals in her urine. The vet said she needs to be put on this special prescription diet to control the crystals. Its Royal canin urinary SO or something like that. Not only is it insanely expensive, but its really low quality. The first three ingredients are rice corn and chicken fat... I was feeding blue wilderness food, which is much higher quality. and she is doing really well on the blue wilderness. We used to feed purina and she lost weight and just didnt look as healthy as she does now on the blue wilderness and I hate to switch her now that she is thriving-she was really unhealthy when we adopted her. Does she "need" the prescription food? Can I just feed half prescription food and half the good food I normally get? Does the high protein levels in her food cause the crystals?