On a Netcommunity event registration form you can have one form with required attributes and another form with out required attributes. In the Raisers edge you can only have one set of attributes so if you need them required to enable Netcommunity to pull that through there is no way of stopping that required attribute for a form that doesn't need it.
Therefore in the event record when adding participants for an event it will not let you save the detail with out all the required information (attributes) that apply to a different event.