I spoke this morning with Alan Emrich about our economic model and about this question. He and I are in agreement on our preferred solution (and he also likes out primary/secondary focus system and the idea that you can only build certain buildings on planets with certain foci! yay us.
I stayed out of the DESIGN thread for the most part, but here's my two cents.
I think the idea of 'stockpiling' production is a very bad one. Production represents a capability, a potential, and that potential needs to be specifically directed to build a specific thing by either an AI 'build list' or the player (perhaps using a build list). This idea is a very simple one and most MOO players are already familiar with it. Trying to make 'unused production' stockpile as some sort of currency that you use to pay your spies, to pay tribute, to hire a leader, or what have you, is counter-intuitive and is asking for cracks in the system. For example: if you use PP stockpiles to maintain a fleet, let's say you need to 'produce nothing' (thus adding to your PP stockpile) on three planets to maintain your fleet. If you produce something on all three planets for any reason, your PP 'income' plummets, and your fleet falls apart because you wanted to build a research lab on all three planets.
Using minerals as cash I like better, but I still don't want to do it. Minerals are a natural resource that fuel industry. Should you be able to trade minerals with another empire? Sure. But should you pay your spies in minerals? Should trade agreements have you exchanging minerals with another empire? Should, essentially, the interstellar 'currency' be a raw material that already has another stated use? I think the answer is no.
I want money. We're using it as money. tzlaine asked in the design thread what we're missing if we don't have it. My answer to that is: a unit of exchange whose primary function is to pay for things. Minerals are not primarily used to pay for things; indeed, they already have a primary use, and it has nothing to do with paying for things.
The other side of that question: what does money give us that we don't have already? It lets us do everything in that list in the first post using a logical and elegant representation in the game to accomplish it. Drek named several strategy games that don't have money; they were Chess, Risk, Warcraft, and SMAC. With the exception of SMAC, none of the first three have trade. You don't have to pay maintenance on your orcs; you buy them once and need your food to keep them, and that's it (and if your food goes away, you just can't replace them - nothing bad happens to them). SMAC's solution is elegant, but 'energy' in SMAC really is money. SMAC has food and minerals seperately from energy. It does use a raw material resource for money, but that resource's primary function -is- money. I wouldn't mind calling our money 'energy' and treating it this way; the only difference I see is that, in SMAC, the natural environment affects your monetary production, so we'd have to ask if we want certain worlds to be better 'trade worlds' than others, but that's a question for another time.
Alan says: we don't have too many resources; PP and RP are not resources but output; in a 4X game with trade, people will want it and expect it. While it's true that realism may not be a good reason to do anything, you also want a certain level of believability within the rules you've built for your universe; galactic empires trading a raw material back and forth violates that in the sense that your players will expect money in a game like this.
Aquitaine says: Other strategy games that are more like us than RISK or Chess all have money; the games we cite as influences in our mission statement have money. We can easily have a non-cumbersome means of producing it (add a column to the foci table in the design doc and a focus for 'trade' - boom, done).