On Monday I talked about Power Level Limits, and some of the good and bad sides that can happen when you use them. All in all, I like the system for what it does, but it isn't the only game in town. On the other side of the equation is what most systems use: good, old fashioned buying stuff. Sure, you can loot, pillage, and just find treasure. But I can't remember the last time I saw a game that didn't use point-buy for equipment, and also didn't have a market system in their game.
The beauty of using Cash as King is that it is something that we automatically and fundamentally understand as gamers. When you have money, you can buy stuff. If you have enough money, you can solve almost any problem. Some systems ignore this, but even in those worlds cash has a tendency to be king. Look at character's like Bruce Wayne and heroes like him. Ever notice how the gadget based heroes are usually fairly well off? Yeah, that's because they can afford to have those gadgets.
In the Mutants and Masterminds game I'm in, my character has put 4 points into making herself a Multi-Millionaire. That's it, 4 points and I am at the second highest tier of wealth in the game. At that level, I should be able to buy a private army to solve my problems for me. Hell, I could probably rent Batman for a barmitzvah - or at least Jason Todd. But that isn't how the universe works, and the game shows it. What about in other games though?
The first thing that is good about cash being king is that it rewards player ingenuity. Systems with this market tend to favor what you have/can afford over any power level on the game. Can you afford a Bolter at first level in Dark Heresy? Then you have one, all for you. Yep, that is a big, powerful gun you have right off the bat. Others will just have to deal with that.
The same is true for those who find a way to scrounge things up though, which can keep the inventive player ahead of the power curve. Also, you get an easy motivation for most groups ("we'll pay you lots of money to do this!" or "We'll give you this expensive equipment!"). Try doing those things in other games, and the lack of mechanical benefit can have some groups a bit less enthusiastic about pursuing their next pay day.
Games using this can also often feel more "real" due to the stronger connection to the real world. People valuing money in a world where it doesn't do all that much is strange. People valuing money in a world where they can buy a squadron of tanks? That we understand.
On the bad side, most of the negatives that are obviously apparent come from player behavior and a feel of diminishing returns. A lot of power in these games comes from equipment, and once the world catches up with the player's equipment they start to feel a lot weaker than they may actually be.
Mostly though, cash being king can bring out some of the worst aspects in gamers. Some players will take the time to loot everything because they need every cent they can find to get even more ammo, or even more ridiculously tough armor. Some players will demand that, no matter the story reasons, the most powerful piece of equipment that can be afforded goes to the most powerful character, because they'll be the most powerful together. Big picture, this may be good for the group (why shouldn't the level 20 warrior have the biggest magic sword you can get him?) but it can also cause problems for the individuals. Especially when those groups also start divying up the cash based on who did how much to help.
Essentially, the bad comes from the same bad we have in real life. When cash is king, the rich tend to get richer and the poor tend to get poorer.
Overall, Cash being King is just as viable as power level limits. It has downsides (book keeping, and sometimes people come into a lot of power surprisingly fast, or build up huge arsenals of looted weaponry), but it also has upsides. For games like Dark Heresy, or a modern mercenary game, I don't see why you wouldn't want to do it. Just keep an eye on the cash, and watch your group for setting up certain members to never have their chance to shine.
Also, if you're a player in one of these games. Don't be afraid to claim the big awesome for yourself, even if you're not the biggest baddest person in the group. Why shouldn't you have something to make you feel cool on occasion too?