The poor heat sink mounting design that everyone seems to have, especially Intel, is not because the CPU is in a socket, or a card in a slot. It is just bad engineering, and Intel could do better. Can you imagine trying to put a heatsink with cooling pipes, a huge radiator onto a card? The card would have to stick out of the motherboard quite a long ways to get some of these on!
Then, rather than the whole appartatus sticking straight up off of the motherboard, it is going to laying parallel to the board taking up all kinds of room directly above the card! I don't know, but I can't see any advantage to having CPUs mounted on cards.
Andno matter how idiot proof you make it, there will always be some fat fingered buffoon, like me, who will manage to break or screw it up somehow if you are not careful and pay attention to what you are doing.
Someone would surely try to plug the card into the wrong slot, I guarentee it.