Any “port” or “jack” or “interface” or “anything” is recognized as a wound by your body. And it will throw all its resources trying to close it, keep it from getting infected, and reject it like a belly piercing done too shallow. This has the effect of compromising your immune system — if your body is so busy with wound care, your allergies get worse, your chances of getting sick get higher, and you get to spend several spare moments every day rubbing the damn thing down with neomycin.
Then, of course, there’s Moore’s Law. … Imagine you could have gotten a cybernetic implant in, say, 1998. Right now you’d be walking around with a PS/2 to USB dongle in the back of your head … you’d be stuck playing Tetris while all your buddies are rawkin’ COD4.