Excellent if it could be done exactly this way. However, things are not always that simple. If a newline is inserted at some point for wrapping purposes, it is desirable and usual to remove what was whitespace around that point, so we do not have unwelcome spaces at start of the beginning line, or spurious trailing whitespace at end of the previous line. If the wrapping device otherwise replaces sequences of many spaces by one, it should be careful at replacing many space by two, in context of full stops.
Emacs does it this way because you reformat the same paragraph over and over. The downside is that sometimes a line is shorter than it could be because it would end in a period. For what we're doing here (producing tidy output) I prefer not to do the Emacs fiddling. --Guido van Rossum (home page: http://www.python.org/~guido/)