If such a standard aluminum heatsink is used 'hotspots' could occur which simply are area's of the CPU-die that are a couple or even 10s of degrees higher in temperature than the average CPU-die temperature. These occur because the aluminum heatsink is not able to dissipate the heat away from the CPU-die quick enough. This is partly due to the thermal properties of aluminum, it can only dissipate a given amount of heat over time for a given contact area, or simply there are limits to the amount of heat you can dissipate through the area of the heatsink that touches the CPU-die.
Many of the large aluminum heatsinks that you'll see advertised might be able to handle 100 or more watts of heat load, but only if we use the entire surface area of the base of the heatsink to dissipate the heat and not the 5 or 10% that you'll actually use with, for example, an AMD Athlon's CPU-die. Intel has already done some of the groundwork with their Pentium 4 as it has a copper-alloy 'slug' that already increases the surface area that touches the actual heatsink, something we recognize from the Pentium MMX and Pentium II CPUs.
Fig 3. The copper alloy slug as featured on the Pentium 4 CPU, increasing the surface area of the actual CPU-die.
Therefore using a copper inlay or whole copper heatsink on a Pentium 4, but certainly on a high-end AMD Athlon CPUs is actually a requirement if you want your system to be running stable and without any thermal related issues. Copper is not only better in conducting heat it is also much better able to handle higher heat loads on a small area, such as a small CPU-die. If the heatsink features a copper-inlay then that inlay will evenly distribute the heat from the CPU-die to the base of the aluminum heatsink which will then be used more efficiently than if it had not featured a copper-inlay.
Heatsinks made out of copper are a nice solution, but are heavy, costly and if an aluminum/copper heatsink is engineered properly they offer the same performance, although a copper heatsink of a similar design will have a smaller size for the same heat load rating. Most of the current copper-inlay/base heatsinks such as, for example, manufactured by Alpha Inc. or EKL AG offer more than adequate cooling performance to handle heat loads up to 80-watts.