I have a reasonably pragmatic attitude to the use of IDNs. Simply speaking I do not believe that they add anything useful. It seems to me that to allow them is an attempt to satisfy the language purists and those keen on having everything PC.
A quick straw pole resulted in no one I asked knowing how to enter any of the macronised vowels from their computer. I think this is a major factor. How does one address an email or enter a URL ? Would all organisations need to register two domains so that they can be found ?
The display of such macronised vowels is also a problem as not all computers have been setup to display them correctly, as an example, the letter I received from the DNC had the following in it 'spell Māori words'. I may not have thunderbird correctly configured but I daresay many people would be in the same boat. I think M&#257;ori is far worse than Maori.
We have got on fine so far without them, my vote would be that we don't need them.