The problem is that the extra fuel needed because of the additional weight of the vehicle might well exceed the fuel saved by transferring energy from braking to acceleration.
Wikipedia states that KERS adds about 25Kg to the weight of a vehicle. On a long motorway run (with free-flowing traffic) that could add quite a few pounds onto the cost of the fuel used, with only a penny or two in savings by return:
http://en.wikipedia.o...ergy_Recovery_Systems
Chris
PS: Incidentally, the hard drive in your computer actually uses a similar system, when it's powered down, to transfer the energy from the spinning disk to the arm carrying the electromagnetic reading head (so that it doesn't crash down onto data-bearing areas of the disk).