Just from reading this thread I notice several things that are helping people form their opinions.
1. Experience. I notice that those who have used a phone with the sensor on the rear(Android) have all been supportive.
2. Ease of use. These people are saying it will make the phone more difficult to use, Awkward, case, or extra steps.
3. Design. It's ugly ('nough said).
Addressing each one individually.
1. One you are right. Not a big hassle in some ways more convenient.
2. The device will be an OLED with that technology Apple can illuminate individual pixels on the screen similar to the current s7. What this allows is if you're phone is sitting on the table where that pesky touch id is on the rear the screen could be already be displaying important information without bed to touch the device (privacy of app notifications I would assume can be adjusted). Meaning if you see you have an email as you lift the phone up to read it you would naturally unlock it. However this fingerprint scanner does look a little low and should be higher on the device. The pixel has cases built for it, sensor on the rear. After roughly a day of use you will find the sensor becomes a natural point of contact on the phone when holding it. I use a nexus 5x even when coming from my iPhone 7 I always instinct place my index on that sensor as it is just natural to hold the device that way.
3. Nothing would make you happy