This is an interesting advance, although at present improvements to our daily lives are a long way off. An immediate benefit is to have a greater understanding of how genetic codes work, which is a worth-while exploration. But alien proteins and whatnot would require not only writing the genetic code for it but also redeveloping the entire protein manufacturing pathway to accommodate for the artificial nucleotides and artificial amino acids. If they do get that working (which would likely require developments even more revolutionary than these nucleotides), though, then we have a potential boon to society.

At present, safety is not a concern. As it says in the article, the organisms can not produce their own artificial nucleotides (because their artificial). This means they will either die off or revert to normal if they can not attain those artificial nucleotides. Even if they were given the ability to synthesise their own nucleotides, I can not see them being any more of a threat than other GMOs with the natural genetic alphabet.