How many people spend any substantial portion of their days away from a relatively large place to put a computer-like device?
I don't. I'm either in a car or in a house, usually. When I go jogging, I need a cell phone, perhaps, and that's about it. I don't do a lot of typing on the run.
So, when the real future gets here, I think all we will need to bring with us for most purposes is an eye or fingerprint for identification. We'll just plug in to whatever is wherever we are. A cell phone might still come in handy, but a do-it-all portable device? Why? We just need built-in computers everywhere.
This is a bad idea unless you know of an algorithm that lets you trust just any ol' input device. If so, please share. Otherwise, always bring your own.
Memory is becoming really small these days, and it gets easily replicated online. Why not having the memory and processor with you and using the convenient peripherals you will find when moving around (screens, speakers, keyboards...)