Uh...
1. In the hatch example given, square ends would look just as bad (ragged) as the round ends.
2. For the OP, for the occasional thick lines desired with square ends, you can use the double line tool to make polygons, 0 thickness line wt., and fill them. Also lets you edit the outlines (say, when the lines don't meet at a right angled corner - square ends would look worse than round ends).
Having the outside boundries of lines at corner meet at a point is something I thought Postscript could do.