I have never heard a solid argument stating why this user would be better off with a Mac which clearly will cost more then a flimsy palstic Dell.
If you're in the class of user who would need something in the class of that studio XPS or above, I can't think of one until you reach the actual need for a workstation except for the OS factor.
If you're in the market for a family computer, I can think of a lot of reasons an all in one makes sense. Recently my parents old HP finally crapped out giving me the perfect opportunity to unload my 18-month old ALU iMac on them. The old HP was wire city. Power cords for both the tower and the display, power and input cords for crappy 2.0 speakers, DVI cable, ethernet cable, mouse cable, keyboard cable, and the power and USB cables for the printer. That's ten different cables, eleven if they had a webcam.
The display, computer, and (much better sounding) 2.0 speakers are in a single unit reducing five cords to one. Webcam is also included. It has 802.11n wireless networking (that's finally fixed in 10.5.6), so it doesn't need the ethernet cable. The keyboard has a built in unpowered USB hub so the mouse connects to it using one port instead of two. You can even save the use of cables all together with built in bluetooth. The printer requires those two cables regardless. It also has a card reader/ 3-Port USB2.0 combo with the USB cable and a power cables. This also was mine, they could have gotten by with just a bus powered card reader.
That's six cables instead of ten. It doesn't tell the whole story though, as the cables are managed in a much better way. Only the power cords left the top of the desk. No cables going back and forth from the top of the display to the tower. In fact, if they really wanted to, they could have used an writing desk in the living room instead the computer desk.
No we come to usability. The slot loading DVD-writer and on keyboard eject button are more convenient from them. The slower write/read speeds aren't a big deal to them. My teenage sister and friends love front row and using photobooth and iChat/Yahoo video chats. Switching users is fast with only having to choose a name in the toolbar menu. Everything is nice and convenient for them with no having to reach under the desk to insert a CD/DVD and no having to remember to turn on the separate pieces. It's also dead quiet. I've heard the fan go on exactly once in a year and a half. The only thing I think it needs is a low end sub-$1000 model with 9400M integrated graphics to replace the spot once held by cheaper G3 iMacs and the eMac. The entry level price is a little high for a family machine.
So, if its a great machine, why didn't it work for me? The same traits that make it such a great family machine work against it in the high end consumer/ low end professional (aka prosumer) market. There's not a lot of headroom in the design and the internals are hard to get at. Apple has a tendency to think in absolutes and not realize the gray areas and differing needs that exist between consumer and professional.
