There are lots of reasons. For one, expanding the genetic code would allow new and interesting structures in RNAs, would allow massive expansion of amino acid c

There are lots of reasons. For one, expanding the genetic code would allow new and interesting structures in RNAs, would allow massive expansion of amino acid coding sets, different evolution paths based on codon usage, etc. Generally in synthetic biology we want to build new systems from scratch, and these base pairs are a wonderful step in humanity defining life instead of life defining humanity.