Leosard: Is there another editor that you use that has the behavior you want?
As Clark asked, when you insert text in the middle of the line, making the line too long even though the cursor remains within the margin limit?
What if you insert a word - causing a wrap - then you backspace to erase it -- would you expect the line to be unwrapped?
There isn't room in a plain text file for meta data or such to track where the editor has wrapped text vs. the user.
So after wrapping the last word on the line to a new one, it won't know that it should wrap the next word into that line.