Definitions:
BitShare - A 'dividend paying, bitcoin-like, crypto-currency'.
crypto-bond - an IOU denominated in one asset and backed by either BitShares or a crypto-bond denominated in another asset.
crypto-USD - a crypto-bond denominated in USD with collateral in the form of BitShare's held by the block chain.
crypto-EUR-USD - a crypto-bond denominated in EUR and backed by crypto-USD.
crypto-BS-USD - a crypto-bond denominated in BS and backed by crypto-USD
Use Cases:
1) Grandma wants to earn a 10% return on her dollars, but is unable to use a computer. Her grandson decides to help her out, so he receives the dollars from his grandmother and then uses them to buy crypto-USD. He buys his crypto-USD by posting an ad on craigslist saying that he is looking to buy some. Someone responds to the add and they meet up to exchange crypto-USD for paper dollars. The grandson then prints out the private-key and gives it to his grandmother to hold on to.
2) A year later Grandma wants to get her dollars back, so she contacts her grandson and gives him her private key. The grandson then gets on crags list and sees an ad for someone near by who wants to buy some crypto-USD. They meet up and trade and the grandson then gives the money to his grandmother.
3) George lives in the middle of nowhere and the nearest person with crypto-USD is over 2 hours away. Fortunately, he has family who live in a major city, so he calls up his brother and offers him $10 to buy $1000 worth of crypto-USD from another local. His brother agrees, so George sends him $1010 and his brother then buys 1000 crypto-USD and then transfers (via BitShare) those crypto-USD to George in the middle of nowhere.
4) Sam is a trader who specializes in crypto-currencies. He has been watching the market and sees that BitShares are 'undervalued' relative to crypto-USD and so he decides to 'short' crypto-USD by selling it into the market. To do this he is giving up his dividends. If he was right, the value of crypto-USD will fall (relative to BitShares) and he will be able to buy back the crypto-USD for less and make a profit greater than his dividends. If he was wrong, then the value of crypto-USD could rise and the network could cover his position causing Sam to lose money. As a result, it is only profitable to short if you expect the price to fall at a rate faster than the dividends you give up!
5) Alex is an early adopter of BitShares, he has mined 100 BS and believes that they will triple in value in the months ahead. He could simply hold his position, but instead he wants to leverage up. As a result, Alex shorts crypto-USD. If he is right, then he will gain more from this short position than the dividends he might have earned by simply holding BS. In this way Alex is creating crypto-USD for the 'risk-adverse' in the early days when BitShares are already appreciating rapidly and paying a very high dividend.
6) Alice is a currency trader who wants to play the EUR/USD market. Alice mines some BitShares and then uses them to buy crypto-EUR. She then uses her crypto-EUR to short crypto-USD (creating crypto-USD-EUR) because she expects EUR to go up relative to USD.
7) David thinks that BitShares are overpriced in a new bubble, so he buys crypto-USD then uses it to short BitShares creating crypto-BS-USD.
Andrew, like Alex, believes that BitShares will go up in value; however, he doesn't like the exposure of going short USD, Gold, or anything else. Instead he deciders to go long crypto-BS-USD. Because crypto-USD pays 2x the dividends (in BS) as the equal value in BS, and crypto-BS-USD requires 2x margin held in crypto-USD to back the crypto-BS-USD, then Andrew can earn 4x the dividend rate by purchasing crypto-BS-USD without having to expose himself to any other currency.
9) Fernando runs an online store and wants to accept payments via a crypto-currency. Unfortunately, all of his suppliers price things in USD. Fernando instead opts to price things in crypto-USD. As a result his customers get price stability, Fernando avoids all exchange fees that would be incurred if using something like Mt.Gox merchant services, and Fernando gets to earn interest while he waits to cash out.
10) Luke has some Bitcoins and wants to buy crypto-USD, using the BitShare exchange. First, Luke must acquire some crypto-Bitcoins so he finds Charles who has crypto-BTC and is looking to get real BTC. So, Luke and Charles use this algorithm:
https://en.bitcoin.it/wiki/Atomic_cross-chain_trading to exchange Crypto-BTC for BTC without having to worry too much about the 'exchange rate' as it should be about 1:1. This 'feature' for cross-chain-trading would be 'built-in' to the BitShare client where Luke and charles would only need to specify the 'addresses' in each chain and the exact exact exchange ratio. The clients would support 'broadcasting' bids/asks to negotiate 'real-time' counter-parties and all transactions would 'expire' within 20 minutes. Because the trading range on BTC to crypto-BTC is very small the market should be very liquid and very quick and can easily require all nodes to be alive and interactive without having to worry about a 'thin' market. It would likely work even if only 2 people were online at the same time.
Given the above 'cross-chain' trading algorithm, I believe that it would be entirely possible to allow much faster trading between all of my crypto-currencies provided all users involved in the trades are 'online' and are not taking out a 'short' position. Only the process of creating a new short position would require going through the built-in exchange. Any trades outside of the 'slow' multi-block process would be restricted to valuations between the current bid/ask price in the block chain.
Conclusion:
I believe that BitShares would enable all of the above interactions without the need for 'market coin'. When dealing entirely with crypto-currencies, no escrow would be required to get BTC,LTC,NMC,etc in/out of BitShares. Once you have crypto-BTC it would be easy to do a lot of exchanges between any other crypto* sub-currency without having to always find a real-time counter party (like MarketCoin would require).
With this latest setup I believe I have 'unified' MarktCoin, BitShares, and BTC Luke's crypto-bond system. I have significantly simplified the ability to find local people to trade with by minimizing the buy-sell spread between crypto-USD and USD and demonstrated how someone like Grandma can use a trusted (decentralized) 3rd party to gain benefits from the system without even having to use a computer.
The only peace missing from this system is 'escrow' for long-distance trades between crypto-USD and USD. As this piece doesn't involve a 'trading engine' I think it falls outside the scope of a p2p exchange. I suspect that companies like BitInstant which already are licensed would enable such a service to complete the overall solution. BitInstant would not be a requirement, but would serve to help while they are legal.