I see nothing different, what's exactly the biggest difference if you pay tax using fiat and Bitcoin? I think you might end up paying 2x tax, first is your income tax and second is capital gains tax.
Technically people need to pay tax if they sold their coins at profit, it doesn't have to wait until Bitcoin can be used for paying tax.
I actually sees it as a major plus to the Bitcoin ecosystem, if most states start accepting Bitcoin in tax collection, because it will definitely burst it popularity among local and by doing so, they are advertising Bitcoin unknowingly to the general masses, which will make it adoption process worldwide very fast.
But the major disadvantage about it is that you might end up paying even more than person paying with the fiat, because after paying your tax with Bitcoin, you will definitely be pay the higher transaction fee attached to it, which will be very much expensive compared to the fiat.