Sorry if this has been mentioned elsewhere... Using multiple cores is more invasive (when doing long renders). One can hit pause, but I'm assuming that memory has still been chewed up and is sitting around in an awkward hard to use state (certainly not a state that would survive a boot). Once in a while if doing a long render, it would be wonderful if there was a way of saving the state of all memory and have a point of recovery. I appreciate very much having renders go "twice" as fast, but the sacrifice made is a greater commitment to the computer.
But even better, imagine if the existing multi-core concept was driven forward with the idea of restore points and (if possible) a continuation of the divide and conquer point of view including a put humpty-dumpty back together again function, then it also might be possible to use a render-farm approach that would/could allow hundreds of PCs to attack a given render.
Imagine the satisfaction of contributing to a massive render that was proto-typed by a Terragen grand-master.