So you start with $1780, spent $890, earned $270 through capital gain, end up with $1160
Tax for capital gain $270 has to be paid
If you always spend bitcoin to buy things, you don't need to pay any tax, since there will be no capital gain
This has a very interesting indication: People will have motivation to spend the coin directly to purchase things instead of exchange it (paying capital gain tax), which also means the merchant can expect a higher sale price if paid in bitcoin (since the consumer are willing to pay more due to saved tax cost)
Well .. ye .....
The whole POINT of Bitcoin is to spend it AS Bitcoin ... not to use it as a speculative/profit toy ...
(well, maybe not the whole point, but a very significant point)