This isn't a black and white scenario. I use AP for probably 50 miles of my 60 mile round trip commute. Here's how it breaks down for me:
In the morning, I leave at 5:30am. There is constant moving traffic at 60-80mph on the first leg of my trip (6 miles). I drive manually, or with AP, and I pay full attention, 100% of this time. If I'm using AP, it's because it's at least as reliable as I am staying in lanes.
I then change interstates. I do this in manual mode almost every time. AP isn't great at dealing with changing multiple lanes quickly and tightly like traffic often requires. Once I'm on the new interstate, I get into my intended lane (second from the left), put the car in AP, and we have what you'd consider stop and go traffic for a few miles. At this point, I open the can of soda I brought. I keep a hand on the wheel, and I pay attention, but I'm more relaxed than I was earlier, when I was doing 60-80mph. At this point, the only thing that I need to do is to respond to someone jumping into my lane and cutting me off (which the AP deals with, but I have more faith in my ability to slam the brakes), or road debris, which at this speed, is not a problem that needs less than a second of response time.
There's a slow steady 40mph drive that I'm in full AP for, drinking my soda and paying attention, and then the traffic thins out, and I notice that I'm starting to lag behind cars, because my AP has a set max of 70MPH from back where the speed limit was 65mph (even though I was only going 20-40). At this point, both hands on the wheel, full attention, and I increase the AP max to 75 or 80, depending on how much traffic I'm in. I switch it back over to manual to make the lane changes necessary to hit my exit, and I'm manual until I park my car at work.
On the way home, I'm in stop and go traffic for an hour. When I'm 'going', I'm going 5-10mph. I am in full AP mode 95% of this time, and I could take a nap at this point, and it wouldn't actually be unsafe. I'm safer with AP than manual at this point, because my attention fades if not and I could drift lanes, or bump the car in front of me. Which I've seen happen to other people countless times, and which just increases the amount of time everyone else is in traffic, too.
Even the most egregious lane jumper can't get into my lane too fast for AP at this stage of my commute. I just set the follow distance to 2, and listen to audio books while I browse twitter or facebook. I look at what's going on out the windshield, but it's virtually unchanging. Like the thousands of people surrounding me, I'm slowly creeping forward, waiting on the 20-30 miles to pass. For an hour and a half.