There are a few games out there (FUDGE was one of them) where game currency (fate points, bennies, actions points, drama dice, whatever) are also experience points. I do not like this model at _all_ because it presents players with a decision that has such a profound opportunity cost as to be no fun at all. If you spend the point on a roll, you've "wasted" it, but if you don't use it on the roll, you might get hosed. This sort of model seems to have its roots in the days when a character could die from a single bad roll, so the calculation was more "If I don't spend this now, I may never get to spend it at all." I don't play games like that anymore, and I'm happy to not have to think that way.
The biggest culprit in this for me was 7th Sea, a game I have a profound love-hate relationship with. Stylistically, it was exactly the sort of game where you would expect the currency (drama dice) to be spent hand over fist in feats of derring-do, but the incentive was to hoard them as aggressively as possible. I ended up inverting the system and saying that *spent* drama dice turned into XP, and that got the spigots flowing again.
As I was thinking about TSOY's keys, I realized this model could work very nicely for Fate as well, if you were to run an XP heavy game. You can treat Fate points as XP by keeping a bowl in the middle of the table. Every time a player spends a FP, it goes into the bowl. The GM might also randomly toss into the bowl when he wants to reward general awesomeness. At night's end, the points in the bowl are converted to XP and divided among the players.
This is a bit of sleight of hand, but it speaks to the kinds of behaviors one wants to encourage in a game. If the currency of the game is something you want to see used, then set up the rules to encourage it. If you worry that currency will be unfunny or game breaking there's an instinct to impose artificial limitations. Don't. Instead, ask if it's what you really want.
4e's action points raise this question quite effectively. They're very limited because their function (allowing an extra action) is crazily unbalancing if it's allowed to stack. Because of that limitation, the flow of a potential non-XP reward is strongly curtailed. In contrasts, consider AP's as presented in Eberron (3e) , simply granting one or more D6's of bonus to a roll. It's useful, and can even help land some big hits, but stockpiling and multiples are much easier to handle.With these, DMs could hand them out for good roleplaying or bringing snacks or for engaging aspect or belief type mechanics.
While I'm specifically talking about XP, currency and rewards what I'm really asking is whether a mechanic is going to get used in a way that makes the game more fun, or if it creates an unnecessary barrier to play.
1 - Give any extra to whoever the table considers the night's spotlight player, or just let it ride til next time.