Thermal pads suffer two major drawbacks...
First they have to be held on the heatsink somehow. This is generally done with gum, not unlike the stuff used on scotch tape. While they do use an extremely thin layer, the gum remains between the CPU and Heatsink, and it has truly lousy thermal qualities. In fact, if you look closely after removing a heatsink you will see that the only thing remaining is the gum itself.
Second, the pads have thickness. In order for the CPU to mate up with the heatsink, this thickness must be eliminated somehow. This is done with heat, by letting the CPU get hot enough to melt it's way into the thermal rubber material, which takes takes place at temperature the CPU should never reach... 80c to be exact. So for a couple of seconds the first time you fire up your system, the CPU is essentially insulated from the heatsink until the pad melts. Hopefully this happens within the 5 or 10 seconds it takes an AMD cpu to self-destruct without cooling.
I always take the thermal pads off and put on thermal grease. Any thermal grease is going to work better and be safer than those silly pads.
--->It ain't better if it don't work<---