The problem is that all the advantages of Bitcoin is now lost, and you have transferred control of Bitcoin to the Bitcoin bank.
For example: you can gather Flexcoins, which allow instantaneous transfers and so forth. But by doing so, you no longer have Bitcoins. You only have Flexcoins, and if the Flexcoin bank goes under... you lose all of your bitcoins.
IE: Using Flexcoins (or any other form of Bitcoin bank) completely defeats the purpose of Bitcoin.