Relatively light simplifies controls for games that require the use of the stick and the PS Move. However we can not really say that the accessory is essential since only Dual Shock held with the left hand may very well do the trick for some games (eg AR5 which actually requires only the stick and neglects others buttons).
For other games like Heavy Rain (Move demo available) we understand the interest of the device which becomes useful since all the buttons are properly solicited.
30 In short it's a purchase thinking. I would strongly ask you the essential question "how will I use game" and see the different opinions that will give you the answer.
Finally ...
All this applies if you are right handed, left handed unfortunate for you purchase is necessary!