Actually, the proposal makes more sense from a business perspective for WGT, as well. If I get the same exchange rate, regardless of my purchase, why would I ever buy more than I need that same day?
However, if I get a bonus (better exchange rate) for buying larger amounts, then I'll consider my long-term needs and likley buy more. Of course, once I have all those extras, I'll want to spend them. Then I run out, and need more. Repeat the cycle.
In the end, I get to play with better tools for a better price, and WGT gets to play with more of my money.
And this scenario doesn't even account for the players who will buy large pots of credits, then get bored and quit. They would likely never do that on the current system.