You and I (and many other photographers on both sides) have very different ideas of what "reliable" means. And this is perfectly fine - that's why both types of modes exist.
The settings that the camera guesses on can change wildly, depending on where exactly you're metering. Your shots will not always be consistent. If you're using evaluative metering, changes in things like the background can have dramatic effect on your exposures. If you're using spot metering, changing where you're metering even slightly can also dramatically effect what the camera thinks is "right".
I guess my main point is that the camera does not have an intuitive sense of what I'm photographing. It can only guess, and allowing it to guess gives it the opportunity to guess wrong. Will I guess wrong too? Absolutely, but it trains me to keep an eye on the histogram and adjust. Exposure will never be something that you should "fire and forget" unless you're taking snapshots. (Again, this is my thinking, not what everyone should think.)
Your bulb mode comment is amusing, but entirely misses the point. The camera is good at precisely exposing the sensor for a specific time with a specific aperture. Nobody disputes this. What is disputed is whether the camera can automatically expose every frame properly. Some people think it can, and they use Av/Tv. Some people don't trust it completely, and they use Manual. Nothing wrong with either.
The question the OP was asking was, why should he take 30 seconds setting exposure manually when in the majority of situations, AP is working for him. My contention is simply that there's nothing wrong with using AP, or SP for that matter, in that majority of situations where it gives the same settings as he would have selected in manual mode anyway. In other words, don't assume you're doing something wrong by using AP or SP, just because other people say they always use manual. Using AP doesn't necessarily equate to fire-and-forget exposure, or to snapshots. As the OP said, you still have EC, spot, etc. to adjust for specific lighting scenarios.
What metering technology can't do, of course, is to second-guess anyone's intention to expose for creativity rather than for an evenly spread histogram, so thank goodness for manual mode. I'm not against manual mode, I just find manual-mode evangelists a little irksome. I'm not suggesting you or anyone else here is in that camp (your views seem balanced and were well put) but there's no doubt they exist. My photography tutor was one - I remember how he cursed when I replaced my manual Yashica with a Contax 139.