Soft drinks basically contain sugar and water. They are not considered acidic. Sugar causes an increase in plaque formation on teeth, which leads to cavities, but other foods like starch have been found to be worse than sugar.
Soft drinks are high in phosphorus, which increases calcium losses by the kidneys. Your kidneys regulate the phosphorus content of your blood and unfortunately, when phosphorus is excreted, it takes calcium along with it. Meat (which athletes sometimes eat too much of) is also high in phosphorus. Read the calcium topic for more info.
Soft drinks because they are high in sugar (12 ounces has 9 teaspoons of sugar) draw water from the blood into the gut to dilute the sugar concentration of the soft drink. This will increase the blood solids including red blood cells that transport oxygen. The net effect is dehydration, which can reduce endurance by 30%, but does not affect the red blood cells ability to carry oxygen. The oxygen content of your red blood cells is dependent on iron stores, your lung capacity and your level of training. Trained athletes take in more oxygen (measured as VO2 Max.)and carry oxygen better in their blood than persons who don’t exercise (couch potatoes).
I would not agree that soft drinks negate the effects of cardiovascular training, but I would not recommend soft drinks prior to, during or immediately after training or competition. In fact, not drinking enough water during training or competition would have a similar dehydrating effect and lower endurance.