For my CPL GNAV the only way to get adequate accuracy was to use dividers then reference against the chart itself.
However, out here in the real world, where I'm trying to determine the best VFR route from A to B, with a minimum of dog-legs around danger areas and controlled airspace, then a long straight edge is definitely the optimal paper-based tool (there are clearly some great software tools available as well, and as it happens I've just ordered the latest version of Navbox Proplan as well). And running at the world at somewhere between 90-110 knots, an accuracy of about a nautical mile is perfectly good for all VFR navigation.
Of course, I could just use a 1m school blackboard ruler or a bit of wood from B&Q, but it's much more useful to have something with nautical miles at 1:500,000 on it - to save ALSO having to use the dividers, bit of string, etc.
Aircraft Spruce seem to have dispatched my tape measure, so I look forward to having a play with it.
G