.

I want to think I've read somewhere its something to do with keeping users from cracking the dies (bit of an issues in the Athlon XP/P3 days). They may very well help spread the heat load a little but, I'm not sure.

What I'm saying is they could make a bare die and build the heat sink out of high performance material to yield superior thermal performance. Essentially build the IHS into the heat sink while eliminating a thermal interface.

Contact between the die and the heat management system is critical and the IHS is a way of guaranteeing contact is adequate. This reduces problems with installation. The IHS provides predictable lateral heat conductance which is not subject to (sometimes inferior) heat sink design. It also prevents mechanical damage to the core. These are just a few of the advantages I can think of to using an IHS.