We've seen many cases where half the recommended mileage was about right. To say that the rest of the secondary ignition system is important would be a formidable understatement. Sure, new, properly-gapped plugs take some of the load off the dialectric capabilities of the cap, rotor, and wires, but they can't correct real problems. Especially in the winter when damp, cold weather tries to coax voltage into escaping rather than going to the electrodes, these components are sorely tempted to cause trouble. Checking them carefully under a good light, spraying them with water and watching for electrical activity, and by passing a grounded screwdriver over them can make the difference between driving merrily through the winter and having to be towed, so this should be a mandatory part of every tune-up.
Timing certainly deserves mention here, too.
It's pretty self-evident that this operation has to be done on an engine that uses points to control its primary circuit, but you should never fail to check it even on those that use transistors instead. Make sure you know what you're doing before tampering with this setting on a vehicle with electronic spark advance. For example, on some older versions, the vacuum line must be disconnected from the computer before your timing light can tell you the truth about the initial spark lead. On older VWs with an idle stabilizer, you'll have to pull both plugs out of the stabilizer and connect them together. Don't retard the spark if there's any possibility of avoiding it.