Burping v/s bleeding hydraulic disc brakes: is it safe?
You aren't assured of getting all the air out of the system.  If your bubbles are further down and this doesn't remove them, you may actually introduce more air than you remove when you replace the cap.
You won't effect a fluid change this way.
You won't replace the crush bolts and other pieces swapped at the true bleed that are        potential sources of failure.
If something happens and you bump/spill some fluid, you are SOL for replacing it.
If you have a leak that has introduced air, this may not help any. 


Not to say this isn't a decent method to try for a home mechanic as an initial troubleshooting step if you are into it.  It may be done faster, with little to no cost, and may help some issues.  However, you are correct, it does have several shortcomings.