Neverwinter, as an MMORPG, needs a secondary currency. Of course we can break everything into simple mechanics and let it roll, but where is the nice little detail in it? Also, if we make Gold a bit more rare, player must decide where they spend it. Gold as a reward in a loot chest and true free-2-play currency gains a bit of worth. You can’t pay the merchant without playing the game.
The system of the Mysterious Merchant is not only existent to give neverwinter xbox one astral diamonds gear parts a certain base value. It’s also a reward to seasoned and active players. Only using Toals as currency would also enable the big spenders to get the sweet goods of the merchant. But such players tend to play less of the game, which means they need alternate sources of Gold. They can trade items of a certain value with other players, opening a secondary market for those that actively grind and hence own Gold. For the system to work however you need sufficient sinks. Offers in the AH could or even starting profession tasks could cost a fee. Craftsmen need a payment as well, right?