There are undeniable benefits of using Google voice such as free SMS, a single phone number for life and easy to use call control features. It is obvious Google in general is looking to displace carriers as the important part of the consumer relationship. By inserting themselves in-between they eventually become the virtual carrier in the eyes of users.
This is great for users and Google but obviously terrible for carriers and Apple is in a position to limit Google's influence when it comes to the iPhone which explains why Google Voice which was approved for the iTunes App Store a while back is no longer approved.
If you look at this situation from Apple's perspective you understand the incredible subsidies the carriers give users and Apple for iPhone purchases are paid for by high rates and the fact that something like Google Voice doesn't exist. If you allow this application and it becomes popular, all of a sudden contracts need to be renegotiated and Apple starts to earn less money per iPhone sold. This is obviously not a good situation for Apple and explains why there will be limited telephony innovation on the iPhone platform.
Having said that the Cupertino company is so far ahead in the smartphone wars, at this point it doesn't matter if they block some of the best apps from their store since there are just so many other available applications to choose from.