All things being equal, more dense sensors are a trade-off. You can resolve more details, with the cost being issues handling indoor lighting that is more commonly found in homes, as your pixels shrink. The irony here is that I don't think 720p is actually much of a cost saver at this point. The cost of these sensors are in the die size more than resolution, and yields should be very good for small embeddable cameras like these.
That said, is it possible better 1080p sensors exist that can handle the same low light situations the 720p sensor Apple uses that would work? Very probably. But I'd just point out that comparing to a good external webcam is comparing to something with a lot more room, and can use larger sensors to wind up with as good, or lower density than what you get in a laptop lid. Just so much more space.