Arsenate ions are just substituting here if I am reading correctly. Not sure my calling this arsenate-based life is accurate. It seems to be normal life in every way. To use a football analogy, it's like the Patriots losing Tom Brady (phosphorous) and having to roll with Matt Cassel (arsenic). Sure they did just fine, but they would have done better with Brady, and it's not like they made the playoffs that year.
What the paragraph quoted there seems to say is that the GFAJ-1 can grow with arsenic, or with phosphorous, but not neither. It would be a lot more interesting and alien if it were some kind of organism that could not grow with our conventional building blocks and had different, unimaginable mechanisms...instead of one that is the same in every way, but just making the best use of its environmental resources.
Granted we didn't know it was possible to substitute the phosphate with arsenate ions, but that was basically the hypothesis of one of the biologists here. They went to an arsenic-rich lake to pull samples to test it and found evidence that the substitution was occurring. I think this is more of a footnote "...and by the way, this can also happen." It makes the possibility of wild and fanciful mechanisms of life and maybe alien life all the more tangible, though. The exciting part of it is "Well, if they can do that, who knows what other kind of life there could be...", rather than these particular bacteria.
I think this is still a HUGE discovery in the scientific world.
I get what you're saying in your Brady vs Cassel comparison, but I think what is truly interesting is the point that this organism can not only sustain life, but grow in the presence of arsenic with or without the presence of phosphorous.
Sure, if this bacteria had an option of using phosphorous over arsenic, it would clearly be a better nutrient source to use phosphorous. However, the fact that is bacteria can actually use arsenic in the same manner as phosphorous in the multiple mechanisms of the cell, including replacement of DNA backbone to one that uses arsenic, to be able to survive, is a big deal!
Ah science. lol.