All EEA member states have to meet a mimimum protection level of €20,000, so there is already a degree of harmonisation on this. The danger of all countries having the same level is that is would be at the lower end of the spectrum, and individual states wouldn't be able to offer their own citizens more protection.
At the end of the day, it is clear that no government is willing to let a retail bank go under. As we've seen with Bradford & Bingley, regulators and government are making arrangements to safeguard deposits (and therefore confidence) bu selling them off to other firms. This makes the Deposit Protection Scheme almost redundant.
Having said that, I would always make sure my deposits were spread between banks (if I had more than £35,000!)