Maybe it is just a different way to look at it, but I would not say burst fire is to save ammunition, per se. Since it is difficult for a shooter with an M16 or M4 to get off more than 3 rounds in full auto before the muzzle goes off target, there is not much point in holding down the trigger and firing another 10 or 20 into the air. True two to four round bursts can be achieved by trigger control if enough time and ammunition is dedicated to training for that purpose. The hard fact is that the army (yes, and Marines) just can't spend days with a recruit doing nothing but practicing burst control in full auto fire. The burst fire capability is like an automatic transmission in a car - it is a mechanical means of achieving easily what only a lot of practice could do otherwise.
Stray thought: Most folks who talk a lot about how it should be done, haven't done it.
As to "bubba" FA conversions being dangerous, that is not true with a well made and properly installed DIAS or lightning link. Some people have tried to achieve FA fire by filing sears or some such, either allowing the hammer to follow the bolt down or to fall before the bolt is fully locked. In theory, the rifle cannot fire under those conditions, but such conversions are not the right way to go, and under certain conditions could allow fire out of battery, with almost certain destruction of the weapon.
Jim