Cardio is great for your respiratory system... however, I wouldn't make that my ONLY form of exercise.
Strength training plays a huge role on successful weight loss (the more muscle you have, the higher your metabolism and therefore the more calories you burn even while at rest.)
I'm not sure if you're asking if just doing cardio every day will contribute to a well-rounded fitness routine, or if you're asking if doing cardio every day on top of doing strength training is a bad idea.
Personally, I like to allow my body some focus for the day. If it's a ST day, I just focus on ST. The only "cardio" I'll do on a ST day is a quick 5-minute warm-up.
Hope this helps.
Original weight in 2002: 220
Original weight when joining Spark 1/8/06: 149
| current weight: 131.0