Encrypting attribute data adds complexity. Encrypting attribute data has security issues. I don't think that any of these reasons are grounds for excluding attribute encryption. There is not a need to encrypt attribute data. I have looked at a number of XML applications and vocabularies that are currently in use.
Attributes can and do contain sensitive information.
The practitioners need to know this, but for patient privacy the billing department should not see this information. Encrypting attribute data adds complexity. It is our bread and butter. I agree that we want to keep standards and implementations as simple as possible. But attributes exist and they contain data.