Running the line back to the boom gives you a 2:1 purchase on the line, adding some mechanical advantage. Running simply through the cringle does add some friction, but not enough to defeat the mech. advantage. Using a good block reduces the friction, but adds weight and expense.
A single line as you describe would only save you line length to the depth of the reef - a couple of feet for the first, more for the second etc.
But you will find it much more difficult to actually pull the reefed clew in tight (and with sufficient "outhaul") without a winch.