If each PP component were an RPG class..

[blackomen]

I'm thinking more along the lines of Warcraft classes but I guess you can generalize them to all RPGs.

Stocks = Wizard/Mage: They can inflict a ton of damage but are also easily killed.

LT Treasuries = Soldier: They can absorb a ton of damage and protect the mages against formidable foes.

Cash/ST Treasuries = Priest/Healer: They come save the day whenever the battle goes south.

Gold: Rogue: They generally behave a bit independently from the rest of the party but offer great help when you least expect it.

[Tyler]

I can also see gold as a Necromancer.  Nothing beats an undead army when there are fresh corpses in the streets. 

[Pointedstick]

Cash could also be a Bard (D&D 3rd edition here) -- makes you feel better when things are bad, but doesn't really contribute that much most of the time.