Burst was created because under-trained troops were dumping mags at one enemy because they got freaked out. They weren't controlling their strings when faced with multiple enemies. If you have ever shot a FA m16, at 25 meters, you will typically get 3 rounds on the target, and the rest will be off due to muzzle climb. Enter 3 round burst. It forced the troop to pull the trigger again, most likely forcing him/her to sight on the target once again. Personally, I'd like to see semi, burst, and auto. Auto has applications. Burst has applications (intermediate distance where speed is required, but a degree of accuracy is also). Semi works well for longer distances where accuracy is very necessary, and auto is great for CQB where you need to get as much lead at the enemy as possible. They all have their purposes.
However, I chose semi/auto because it's the most versatile. You can get your bursts to 3 rounds if you practice, and you can have semi's inherent accuracy, while still having the ability to go FA if you need.