You would want to do that for the same reason we have a type-system - it provides additional documentation and meta information to developers and to the runtime.
In the next revision of type system fields and methods that have
string serialized X value type will auto generate
parse* methods to parse the string. I.e. in your example it will auto generate:
parseGetUrl: function(failIfInvalid: boolean): Url
p.s. typically it is better to persist / pass around compact string representation of a complex data structure like
p.p.s. please remove the silly
get prefix from methods - you must nor like auto-complete