I'm presuming you're saying that because 2.0 requires an issuer to back his MPA with some kind of collateral, or other insurance? ...Because if there is no cost to issuing an MPA, what's to stop an attacker just creating a bunch of them with the transaction fee set to some insignificant value, and just DDOS the network?
Let's say you want to submit a transaction that needs to pay a transaction fee of 30 BTS (according to the current fee schedule). This might be a transaction that transfers a UIA from your account to another account. Also, pretend your account does not have any BTS at all. If the UIA issuer has some BTS in the fee pool (at least 30 BTS necessary in this case), then it is possible for you to submit this transaction even without any BTS to pay the fee if you are willing to accept the issuer's exchange rate.
Let's pretend the market price of the UIA is 15 BTS/UIA. The issuer may have set a core_exchange_rate of 10 BTS/UIA instead to play it safe (account for market liquidity, volatility, management expenses, profit, etc.). So you need to pay a fee of at least 3 UIA for that transaction otherwise you will not be able to get enough BTS (specifically 30 BTS) from the UIA's fee pool to pay the transaction fee. So let's say you submit the transaction that withdraws 103 UIA, sends 100 UIA to the recipient, and pays 3 UIA as a fee which is converted via the fee pool into 30 BTS (20% of which goes to the reserve pool and the remaining 80% goes to your registrar/referrer). What happens with the conversion is that the 3 UIA is deposited into the issuer's account and 30 BTS is withdrawn from the fee pool.
Now the issuer can go into the market and exchange the 3 UIA for BTS. The market price is 15 BTS/UIA, but because of liquidity let's pretend the issuer only gets 36 BTS. The issuer can then put 30 BTS back into the fee pool to top it off and take the remaining 6 BTS as profit.