This is a clean way to solve the low hanging fruit of form validation, stuff like isItAnEmailAddress().

What I always find messy in web frameworks is doing validations which are more complicated.

For example a radio button choice which hides another form element which would otherwise be mandatory but should now not be filled in at all. Or where supplying a value between certain dates changes another date field from optional to mandatory and also must check that these dates are between certain values dictated by the first date field.