I assume (but I don't know that for a fact) that the text comparison tool is not based on FL having manually highlighted the differences between all possible texts but is an automated comparative tool. Comparing passages across versions is handy, BUT there is much more juicy functionality to be had yet if it can use the same comparative technology: compare two or more passages in the same Bible.
I know the parallel passages resources line up passages side by side. But they do not highlight (as far as I know) the differences between passages the way Text Comparison does. This would be a wonderful addition and an extremely useful tool. Imagine being able to line up two parallel in the gospels and see instantly what the differences are in the Greek text! Or doing the same with doublets in the Pentateuch, parallel psalms, etc.
Unless there is already a way of doing this that I am not aware of...