I'm guessing it would be great for anyone using a computer on which they don't have administrative access, like people in internet cafes, jobs, or schools. It would also be good for people who want to enter some text but don't want to go through the hassle of setting up the operating system's IME, like a first-time language student eager to jump in or a person who just needs to reproduce a few characters of foreign text. I suppose it may also be useful in contexts where an IME isn't available (some mobile devices?) or where cross-platform IME instructions are needed.

Great idea, btw. I have found myself looking for something like this in the past.