Just wanted to add my 2 cents or rather.. keep them. First you could buy some gems to buy a cool outfit and then spend whatever was left on gold. Which made getting the outfit even more worthwhile. Now you would be stuck sitting on gems forever. And if you wanted to trade gold for gems, you could just do some calculations and get exactly the amount you need.
Hope Anet will add the option to fill in a custom amount of gold/gems to trade.