I have the same question, for example, for a walking map, a line drawn along a road seems most efficient, but if the road ends and an unmarked trail begins, the line tool must change to a series of straight lines.  These two lines cannot join to make one, single line.  It would be extremely useful to have these two lines join, so that a total distance can be quickly found.  This way, if alternative routes are being studied, the maps can be modularized and the total distances quickly compared.  :^)