I'm not sure if a lack of fruits and veggies leads to cancer. It leads to malnutrition. But, fruits and veggies have many benefits to prevent cancer: fiber helps your poo move through your bowels better; antioxidents fight free radicals; vitamins in natural form do lots of cool things (metabolism, brain function, etc.) that help your body's cells replicate normally.

I supposed you could argue for multivitamins. My approach is to not bet against 6000+ years of human biochemistry.