Cardio (steady state) is often over used and absued during pre-contest training or with most trying to reduce bodyfat. Trainees think they are doing aerobic activity to raise metabolism, but that couldn't be further from the truth. What aerobic training does is make a person more "efficient" at using fat for fuel. Now to be more efficient at something means to use less of something i.e Fat in this case.

The last thing we want when trying to lose fat is become more efficent at burning it, we want to be like a high powered V-12 engine and burn fuel as fast and as much as possible, not be a hybrid car and burn less fuel. So if you do 5 days of 45 min cardio eventually you will become efficient at it and to get same results you will have to do more. Its the law of diminsihing returns ( so like a car instead of getting 10 miles to a gallon you get 30 miles per gallon).

Steady state cardio ONLY burns off calories, it doesn't do anything to enhance metabolism. When/if its used it should be done as sparingly as possible and after so amny weeks should be stopped for a period of time. If applied properly and in moderation it can be useful in losing fat, but in the bigger picture your goal should be to rely on it less and less. This is were proper programming and sequencing of programs comes in.