Somewhere in Shimano's many published guidelines has been the maintenance "need" to replace shift cables periodically. I don't recall the frequency but 6 months comes to mind. Which I remember thing was crazy, unless you are mega mileage and/or just really hard on your cables.

We have a couple of customers who have told us to replace their cables whenever they have their bike tuned (which is generally once or twice a year for these riders). Andy (who also has fished out a broken off cable head from his shifters, but being an Ergo guy it was easy)