I'm sure real-time rendering will be useful in a variety of use cases. However, one of the main reasons I use LaTeX (when I did) was because it allowed me to separate creation of the content from worrying about the format. This meant I could switch between author and designer without my (feeble) focus being distracted by an image jumping around or an equation overflowing. Indeed I actually found 'compiling' a LaTeX document quite exciting as I anticipated how it might look.
I can see this being useful in the final phases of a paper, when I’m killing orphans (words and sentences), overflows, and underclass, via slight sentance rewriting. Right now it’s modify, build, examine, modify, ...
Of course that I can do this at all is due to tex’s layout algorithms that try for good fits but not too hard (a good choice IMHO).
