UIAs have no pricefeeds
compare it to sending BTC into Poloniex. They call it BTC, but you are actually trading internal UIAs from Poloniex. Till you are sending out, then you have BTC again.
It is the same system, but in bitshares the current available UIAs are called trade(Blocktrades), metaex(metaexchange) and openCOIN(openledger)
in the future more UIAs will join and you can hop between different exchanges instantly and if you dont like the counterparty risk go into pegged assets with a feedprice (counterparty free, pegged with collateral)
would be like instant transfer from bittrex into poloniex, because you see a great abitrage opportunity. over time this will hopefully evolve in bitshares.