We're talking campaigns styles not one adventure where the PCs are busy for 5 straight hours. But even of the published adventures very few of them are like this.
as has come up in the plot threads, I have played in campaigns with very little down-time for PCs. Being in dangerous territory, with things to get done, leaves very little time for other pursuits.
I know rechan is just poking fun at this, ala Murphy's Rules. So I know he's not overly distraught over the problem.
One good reason RPGs tend to work the way they do (XP that can be spend to increase any skill, not just the ones you used) is that tracking skill usage individually would take more work. On a computer, it is trivial, as seen in ElderScrolls. But then, in Elder scrolls, i can max out my sneak skills with a rubber-band and a dark corner...
As some have suggested, it is assumed PCs practiced in down-time, for any skill increase that didn't see use. Obviously, that's an assumption, but one you have to make if you want to keep verisimulitude.
Got read my recent blog articles for not nit-picking rules, and spreading XP over time. They address what rechan is talking about, to some extent.