4/8gb SD cards are very cheap and even the most technophobe teacher can comprehend pulling out the existing card and replacing it with one from the box of pre-prepared cards.
How is a pre-prepared SD card any different than a pre-image Windows drive? Or pulling a hard drive from a HDD cage than pulling an SD card? Its the same thing, only one is way faster than the other, and you can actually do some productive work.
1) Cost. An SD card is very much cheaper than a HDD. Secondly a Windows drive requires hardware capable of running Windows. It requires you to pay the Windows licensing fee. Very much more expensive.
2) My non-computer literate sister, or my 7 year old nephew, understand about SD cards, how to insert and remove them. That they contain data. If I gave them a screwdriver and told them to pull a drive and insert a new one they would be lost.
how about we talk numbers
16GB SDcard = $12.00
500GB HDD = $85.00
the SDcard costs $0.75/GB while the HDD costs a whopping $0.17/GB.
How is a $35 Pi/Linux combo any cheaper than $270 Win netbook? Factor in your HDMI monitor, keyboard, mouse, powered USB hub, decent power supply, some nice case, a decent size SDcard, time spent to make the contraption somewhat portable, time spent compiling/dd-ing your distribution, maybe some more time spent making the USB wifi work, and a lot of time spent waiting for your browser/word processor/etc to load from the uberfast class 99 SD card. Oh before I forget, time spent navigating the web looking for guide on how to patch your kernel, make X work with OpenGL, etc.
I bet your sister/nephew is capable of DDing the Debian image unto the SDcard. Ohh snap, she might as well use win32diskimager to do the same thing with less time spent on researching what dd, mmcblk02, /dev/null, uboot, etc are.