My understanding is that you can use both in the sense of alternating between them, but for any given message, it only appears in one place.
From a user experience perspective, this means that some fraction of users will be confused, annoyed, or downright furious that they now have two completely disconnected ways of making calls, recieving messages, listening to voicemails, etc. I think that's what bothers Apple about it.
I don't understand this point which someone tried to make above. If GV actually came pre-installed you may have a point, but the fact is that the users seeking to use GV will not be confused, annoyed, or furious at all. We understand exactly what GV is and what we want out of it. Apple's point is completely bullshit about confusing users because only users specifically seeking this functionality will have it.
Maybe you do, but can you guarantee that every potential GV user does? Of course not. This is what I mean by "some fraction of users". That fraction is unknown, but as GV creates a significantly different parallel method of accomplishing core phone functions, it is quite plausible that users with an imperfect understanding of how it works and why would suffer as described--for instance, a user who does not understand why his voicemails might show up in two different places. While it is true that it is ultimately the user's responsibility to understand the software he is using, this potentially impacts the core functionality of Apple's device, so it would be only natural for Apple to want to consider it carefully.
Apple's point is completely bullshit about confusing users because only users specifically seeking this functionality will have it.
Does not follow. Users who specifically seek to purchase (say) Microsoft Office are entirely capable of being confused, annoyed, or infuriated by it. The same can be said of most software, hardware, and machines in general. Have you ever seen someone try to push a pull door or pull a push door?
Yes, they're certainly stretching the meaning of "prevent" there.