Ability to represent two controls that depend on each other. For example :
_ Representation of date ranges (date1 <= date2)
_ Representation of interval values (value1 <= value2)
_ Validate this field only if the field 'x' is checked
Thefore the ability to add validation rules to dependant input controls is a must.
A good example of such validation is defined in Struts validators "validwhen"