If you wanted to pay for AdWords on credit, why wouldn't you be doing so already? I assume most businesses are already using a credit or charge card. And getting benefits like cash back or points for doing so.
If you're already doing that, why would you want to switch to a Google card? They offer no apparent benefits aside from a possible higher credit limit. If someone can't get a decent line of credit, would they really spend so much more on AdWords if given a Google card that they'll be more profitable?
If a business can't get credit at all, why would the banks Google partners with extend them some?