Illinois has mandated that all new windows sold in the state be argon-filled. This pisses me off because it has now set in stone one specific technology, and if a better one comes along, we won't be able to use it.
That said, what is the logic behind argon-filled windows. I would think a vacuum would be even better as it would prevent heat from conducting outside the house in winter. If a dense gas like argon is preferable, why not sulfur hexafluoride instead?