There are times... many times... in MTBing when things sneak up on you and you simply need to be in a lower gear and you need to be in it now. There's simply no time and no opportunity to let off the pedal pressure. That's why I don't like low-normal rear derailleurs. You can force a downshift in a high-normal RD but if you're using a low-normal then depending on how much tension is on the chain you may have to wait to get it up onto the gate in order to downshift and there's even a chance that the downshift will never happen. Additionally, the rate of downshifting is dependant on the rotational speed of the crank which in those types of critical moments is usually pretty slow as it is. While this is not as critical in an upshift-necessary situation, in a downshift-necessary one, it may mean that you never get the chance to complete the shift in time.