I just got one a couple of months ago, although an HP, not a Dell.
You seem to think that the "all in one" is related to the computer's capabilities. That's not so much the case. For that, you still have to look at the processor, memory, etc.
What makes a computer "all in one" is the integration of the CPU with the monitor. So you don't have a boxy CPU sitting under or on top of your computer desk any more in addition to a monitor, but just a thick and heavy monitor which has everything in it, plus usually a wireless mouse and keyboard extra. I got it because I wanted something more compact than a full desktop, but with a bigger monitor screen than a laptop. I've got all my peripherals like scanner, DVD burner, external harddrive etc. attached via USB. It works just as capable (actually has a better monitor) as my last desktop, it just takes less space.
I have to add that I don't do games, movies etc., so my use is pretty traditional: internet, Office, pictures, and such.