It's not about the phones learning the patterns, which as you've said recent phones have corrected that, but due to energy sustainability and trickle charging (once it's fully charged, it will keep energy flowing in order to keep itself fully charged) - you should unplug once it's fully charged in order to conserve electricity. A phone may not seem like much, but every bit counts and it permeates throughout the rest of your personality/devices and adds up, especially when everyone does it.

I charge my phone when it needs it and unplug it when it's done. Sometimes that lasts two days, sometimes it's half a day, sometimes I plug it in first thing in the morning, sometimes I plug it in while I'm driving home at night.