Cardio isn't nearly as effective as strength training (think intensity, and EPOC), basal metabolic rate (think what added muscle does to BMR), and diet, at burning off stored bodyfat.
Cardio plays a role when you can't use the above 3 things well enough to lose the amount of fat in the time you need to.
Some guys need to add cardio because their BMR isn't high enough to burn off all the fat they have even with all the muscle mass they have (read: they were a Miss Piggy in the off-season and are carrying 65lbs of extra fat at 14 weeks out from the show).
Some guys use cardio as a way to offset their lackluster starving skills. Some guys just can't starve (can't suffer hard enough), so they add cardio to make up the difference.
And you can only starve yourself so much and lose fat only so fast...so if you're too fat and you need to lose 40lbs of fat in 12 weeks, you're going to need to add cardio. If you only need to lose 20lbs of fat in 12 weeks pre-contest, there's no reason you can't get away with just lifting weights and dieting, if you're good at sticking to your daily caloric intake limits.
Of course, if you really extra extra fat, then you'll need some more help (T3, Clen, ephedrine, GH, DNP, etc...). But that's another story.
And finally, some guys just can't lift weights all the time for long periods of time to burn the fat off. Plenty of guys develop over-use injuries doing 2-3hr workouts 5 times or more per week. Elbows get hurt, knees get hurt, shoulders wear out, the lower back spasms....shit like that happens when you get older. If you use enough junk, you can mask the pain, but god help you when you clean out or give up lifting. You will be one busted up sonofabitch. You can't put 10 tons on a 5 ton truck for years at a time and not expect to wear the thing out.
