It's not that big of a deal. It's just battle grunts and all that for the first hour or two of the game. Probably not even noticeable, which is probably what they were thinking. If it is actually the case.
It's a series of "not that big of a deal"s that become a problem though. While it's true this instance may not be noticeable by some, as previously stated, there should have been no reason for the switch in the first place. The project files are all labeled, so whether they switched them on accident or on purpose (as some oddball Easter egg), the point remains that Jesse McCartney tried to make Ven and Roxas sound completely distinguishable from one another. That could be a little jarring jumping in and out of battle and hearing two different voice types/tones and what have you.