Banks originated as goldsmiths who stored people's gold and gave out paper certificates representing the quantity of gold that was stored. This was a valuable service, because storing gold was hard, and paying people with gold was hard. Bitcoins are very different. Storing them is in principle easy, especially if you have a bitcoin-only device as I advocated here, and paying people with them is easy. All of the reasons for putting your gold in the bank are thus irrelevant for bitcoins.

* People can steal your gold out of your house, and thus it is wise to put your gold in a secure bank. Not so with bitcoins, so long as you encrypt them with a password. Criminals could steal your computers and not be able to access them because they wouldn't know the password. Further, if they did steal your encrypted bitcoins, you could have them backed up elsewhere and thus you would not lose them. You have absolutely nothing to gain from storing your bitcoins in a bank in terms of security. You are actually making things less secure by giving 100% access to your money to a third party who can never care about your money as much as you do.

* It is hard to lug gold around to pay people with it. This is completely irrelevant for bitcoins since you can instantly pay anyone anywhere in the world. Nothing is easier to carry around than bitcoins. Especially with smartphone clients. Banks do not in principle make bitcoin transactions easier. If places like MyBitcoin make anything easier, that simply represents the difficulty of using the standard client. But that is not a fundamental problem - it is a problem that can be fixed by making a more user-friendly client. So again, what made sense for gold makes no sense for bitcoins.

Banks are unnecessary for bitcoin. You do not gain anything by giving someone else responsibility for your money. In fact, given the recent MyBitcoin and Bitomat catastrophes, it appears that not only are bitcoin banks unnecessary, they are harmful. Outsourcing your finances will always be a poor financial decision in the same way outsourcing your intelligence will always be stupid. If you opt-out of your responsibility, you are unlikely to come out ahead. Keep your bitcoins as close to you as possible.

(Note that I am not arguing you should never store any of your money in online services. Just that you should only store small quantities that are as small as your level of trust for those services. If they ever have most of your bitcoins, then you are probably making a mistake.)