See http://lists.jboss.org/pipermail/beanvalidation-dev/2012-December/000684.html:

I think this would be helpful where validation is based on some sort of context which is not expressed in the constraint annotation attributes:

• "Password must be at least {min} characters long for users in role {role}" (role is retrieved from the current user)
• "Order number invalid. Reason: {reason}" (reason is one of several possible ones checked by the validator)

One could work around this by creating the message completely by yourself, but I think it would be nice to leave retrieval of the template etc. to BV and just put custom variables in. In combination with the EL feature, you could also put in variables evaluated in the EL expression.