I understand what you are saying. I wonder though, why couldn't the on die IMC be used just like the IMC on the mobo which intel uses? Nm, I think I just answered my own question. Each additional core has to be hard linked to it, so, you can't just paste another dual core on without having the additional pins for the additional IMC to communicate with the memory. Well, that sucks for AMD. While, the monolithic design might have advantages, it definately has it disadvantages. So, the only way would be to have multiple sockets(confusion, mobo makers would hate it) or, have a one size fits all type of deal with a ton of pins which the low end cpu's wouldn't even use. This would drive the cost of the board up, and those who want to build low end machines would hate it. The IMC is putting them in a bit of a pickle.
wes
Edit: The IMC would have to be designed to be modular, so, I don't think it would be possible to make it part of the core if it were modular(using modular to describe plugging extra cores into it like Intel). If it were part of the core, then every core would have this modular IMC. It doesn't sound like it is possible without having a massive socket. Otherwise, if they were using the same pins, one core would have to supply the other core with the desired data, or, it would swap back and forth or slave like you said.