Another reason for removing the per-resource custom constraints is probably performance - e.g if you create a 1000 servers in a ResourceGroup we make at least 1000 calls to glance validating the same thing, vs (hopefully) 1 if you just validate the parameter via an explicitly defined custom contraint (or not, if you prefer not to fail fast during validation for whatever reason).
Another reason for removing the per-resource custom constraints is probably performance - e.g if you create a 1000 servers in a ResourceGroup we make at least 1000 calls to glance validating the same thing, vs (hopefully) 1 if you just validate the parameter via an explicitly defined custom contraint (or not, if you prefer not to fail fast during validation for whatever reason).