Semi auto fires as fast as you can tap the trigger, they still fire plenty fast. Test out how long it takes you to get through 30 clicks on your mouse. Now keep in mind with the automatic, by your third or fourth shot your aim might be 10 feet higher than where you set off aiming at the start of the burst, and you can see how even in crowds a semi automatic weapon would be far more dangerous as you could control where the rifle is pointed.
