Changing a cat's food randomly (or based on whatever's on sale) is not a good tactic, but having a regular food rotation of 3-5 foods can prevent your cat from becoming so attached to a particular food that he will not eat anything else (which can become a problem if the manufacturer changes the formula to something unpalatable suddenly).
In addition, a food rotation also can protect against accidental nutritional deficiencies due to manufacturing error. An article found on Pawnation reviewed a study that found:
"Since 2009 [article published September 2013], there have been 5 major voluntary pet food recalls
involving thiamine-deficient pet foods in the United States that
ultimately involved 9 brands of cat foods and at least 23 clinically
affected cats. Most of these recalls were instituted in response to a
report from a consumer or veterinarian after treating a cat that had
clinical signs consistent with thiamine deficiency."
(the study is not available on the web, but it is:
Thiamine deficiency in dogs and cats. Markovich JE, Heinze CR, Freeman LM. J Am Vet Med Assoc. 2013 Sep 1;243(5):649-56. )