Paying your way through a game removes the achievement and prestige of unlocking new weapons, cars, and magic swords in a game. These items are there to reward the player for progressing through you game. If a player can not progress through your game, then you have failed on the difficulty curve. Simple as that. (So as long as your testing your target audience anyway

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Paying for progress and items, ruins in game economies and sense of fairness. Because that rich kid will always have the fastest cars, and most powerful guns. This is just a simple money grab. No doubt many people will be tempted to buy a car out of frustration.
When Need for speed shift came out, I thought maybe Foza, GT, and Race Driver, have a new competitor... I was hopeful that this genre would grow and flourish in an era where shooters are just rehashing them selves.
I was also hopeful EA had moved on, and started to make games, for the love of it, but alas they have not changed a bit.
There is another game out there like this... its called 'Evony online' uses the same system. The more money you spend on the game, the faster you progress, and the more "powerful" you become. Apparently its quite the money maker.