Oh no, it certainly helps grandmas and the one-per-classroom public computers (China, 2008-). You get all the USB sticks coming in and out, and before you know it you get that one obnoxious virus that hides all folders and replaces them with a .exe of the same name.

And yeah they do boot from a readonly C: with some magic to make it appear writable per session. But re-infection is quick, especially when you have extra writable data partitions.