Returns the value represented as string - in the best format that we can.
This has to work even without knowing the exact data type. Therefore
there is no guarantee that the returned value will be precise.
This method is used as a "last instance" if everything else fails.
Invocation of this method will not change the state of the xnode, e.g.
it will NOT cause it to be parsed. It can be invoked without any side effects.