Yes, yes, Visual Voicemail is one of the major features.
The main point I was trying to make is that it's much easier to test an equation with few variables as possible. While Apple may be enjoying monthly revenue from AT&T customers I also believe that it was not the only factor leading them to want to stay with one carrier. Troubleshooting a brand new technology is much easier when you have fewer variables.
-Phillystax
My point in quoting your post was that it's the ONLY feature