Well, if a single unit can do what a raid setup(several drives + cables + controller + available internal space inside the computer + required raid knowledge + setup time), then what do you think the advantage is?
I am sure the same questions must have been asked of Thomas Edison, Marconi, or Henry Ford before they came out with their inventions. Maybe your questions are ahead of it's time?
Notice I used "internal storage media", which means it doesn't have to be discs(platters). My line of thinking was, for simplicity, it could be something as hot swappable(pluggable) as a USB stick or an SD card. So it's not the drive that get's swapped, it's the internal storage media that get's swapped, like swapping the SD card in a camera.
I will end my contribution to this particular topic with some of my favourite quotes from "Think & Grow Rich".
"Those who are afraid of new ideas are doomed before they start".
"Without doubt, the most common weakness of all human beings is the habit of leaving their minds open to the negative influence of other people."
"MAN'S ONLY LIMITATION, within reason, LIES IN HIS DEVELOPMENT AND USE OF HIS IMAGINATION."
I guess if someone can imagine a single unit drive with built-in redundancy, and they have the resources and capital to make it into a reality, they'd be called a pioneer. Until then, they're just plain KRAZY!!