1 GM: <sigh> Okay, experience points. Remember my house rule: you can only spend them on skills you actually used in the adventure. I've kept track for you.
2 GM: Alvissa, you rode a horse, haggled with a merchant, practised with bow and sword, maintained your armour, cooked meals. You can improve any of those. Kyros...
3 Kyros: Yes?!?
4 GM: {holding head in hands in despair} ... you can increase the power of your fireball spell...

2014-02-01 Rerun commentary: This plays on an actual rule that is used in some roleplaying games. In many games, your character earns generic "experience" and you can spend it to increase whatever skills and abilities you like, up to the amount allowed by what you have earned.

But some games specify that you can only allocate experience towards improving skills or abilities that you actually used during the period in which the experience was earned. The justification for this is an improved level of simulated realism. After all, if you spent a month sparring with a fencing opponent, you're much more likely to have improved your fencing skill than your skill at playing the piano.

The down side of this is that you (or someone else - probably the poor overworked GM) need to keep track of what skills you have used and practised.

A more subtle effect is that you tend to use skills you're good at, so the lesser skills that you might actually want to improve more may end up the ones you have less chance of improving.

All things considered, this is a rule I think is better abstracted away slightly. Unless you really love meticulous attention to detail and incredibly fiddly book-keeping. (Hint: That's not why I play games.)