If we're talking about items and not the currency. I pay $20 for a mount. I have to sub to use it.The trading cards could not be used as currency inside WOW. They on its own were never outside resource that you could trade after you logged into the game. Its true that they were items that were bought for real money. But real money was never currency AFTER you logged into the game.
There is a big difference. What BLizzard is talking about now is adding direct IN GAME currency that is connected to real life money that can be used to buy in game items. It opens up many different worm holes that TCG could never do.
ANY person that pays a sub for a game should not have to then load extra cash into ingame currency to buy through cash shop. WHy? Because only F2P games use currency directly connected to real money that can be used in cash shop.... Sub driven game does not need such system cause it already gets his revenues through the subs.
There is nothing complicated about this... You pay a sub fee and buy items from web store. OR you buy ingame currency for real money to spend on ingame web store items. Thats why one is called Sub based game and the other classed F2P game.
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I personally think this is very different. As a player that has been playing F2P game with cash store for few years... The core of the game totally changes with connecting any currency with real money. Be that in GW2 or in Neverwinter. Or even RIFT and SWTOR now. Some of them offer sub but none of those games prevents a player that has used ingame cashop to not log into the game. They might close down some content for them ore limit it. But not force anyone to pay for just loging in for wearing those items.
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I personally will not be supporting WOW if they decide to add ingame shop and still expect everyone to pay sub. It simply would not be good long term model and will weaken the game for new players considering the age of the game and what is already being offered on the MMO market.