Allowing WoW Tokens to be used for Battle.net balance creates new demand for tokens. There are some players in WoW with lots of gold who have considerable excess even after paying for their subscriptions with tokens. There is a limit on the amount of subscription any player can use, and that amount of tokens doesn’t make a dent in the gold hoards held by some players. Now, assuming those players want to buy stuff in other Blizzard games, they have incentive to buy a lot more tokens.
Something to keep in mind as well is that this will make enchanting materials more valuable, since it is likely most players will now simply scrap their boe greens instead of getting them disenchanted. Also redundant dungeon/raid gear can be scrapped rather than traded to the (guild) enchanter in the run. Less stuff gets disenchanted, meaning there will be less ecnhanting materials, meaning their value will rise. All the more reason to DE rather than scrap as an ecnhanter.