I have been arguing this for years - in a 10% tongue-in-cheek kind of way - and it's strange to see it written down in an article.
The stock response tends to be that in an ideal world it would be great, but it's not practical. Of course, perfectly decent, well-meaning and good-hearted white people sat around dinner tables during apartheid and slavery and said exactly the same thing. History has proven them wrong, and in fact if we imagine popular support for uncontrolled global immigration, it's rather easy to conceive a practical implementation - quotas gradually increased, borders lowered - it could be achieved in a controlled way.
I also hear the 'why should we?' response. Indonesia has squandered its natural resources - aka it's their own fault they're poor. But none of us would stand in front of a classful of Indonesian schoolchildren and say this. Being born in a rich western country is a colossal slice of luck, not a personal achievement. The fact is that thinking of people in terms of nations tends to dehumanise them, and that's one of the reasons why greater migration and a blurring of borders would be a good thing.
The economic arguments around immigration have raged in the press here in the UK for the last few years. Here at least the general academic consensus has been that it's a net plus, and that the additional burden on state benefits is outweighed by the economic benefits of a well-motivated and grateful migrant workforce. The risk to any benefits system is not more people per se, but that it will be abused - ie the development of a benefits culture, where people choose not to work because the safety net will support them. It's up to us to decide whether this attitude is more likely in the indigenous population or with recent economic migrants.
The real barrier to adoption is lack of popular support, and I accept that few people are going to agree with me on this issue. But then again, there's this: http://goo.gl/95g5kQ