I've been using HV in several projects for the last few years, commonly to validate inputs to a REST API. One common need I've had is a validator to ensure that the items in an incoming array are unique.
Because XML & JSON don't have a set type, JAX-RS always deserializes these to a list. When you then convert them to a set (with, for example, new HashSet(list) ), the duplicate items (same equals) are silently dropped. This behavior is often surprising to developers and end-users alike. It's much better to raise a validation exception, highlighting the duplicate items for the API consumer to correct them - a good use case for a validator!
I have such a validator that I reuse anyway, and would be happy to give something back to the HV project, if you think it would be useful