I will admit that I didn't read the whole thread, but in an attempt to return to the OP's actual topic, I'll say this:

I fly out of ATL. Usually this is very easy, and usually I don't check bags anyway. But from time to time I'm running late, or it took longer to park, or the terminal is very busy (even the Medallion lines) and I am checking bags, or I'm with my family, or whatever.

At these times, the curbside check-in becomes a value-added service, not just Delta doing their jobs. When I use it, I'm therefore happy to tip.