yea, you can't run as fast as you can on a treadmill, but the whole goal of anerobic exercise is to increase the effeciency of your heart/cardiorespiratory system. The whole point of sprints is to get your heart rate up to a max and allow little time for recovery. Eventually your heart gets stronger, which increases your wind and you will need less recovery time between bouts of intense activity. You can do this on a treadmill too, it's all about preference. I personally love sprinting, but I have constant access to a treadmill and don't always have the option of sprinting. Even really intense bag drills are just as good as sprinting. All it is pushing your body to it's max, giving it a little recovery, and doing it over and over again until you're superhuman.