I appreciate the comments, and the other benefits I overlooked.

To state the obvious: upgrades, free and otherwise, mean something only when there's a first class section on the plane. Apply that concept to three real-life examples:

1a) LIT-LGA on DL. I get an MD-88 to ATL, then a 757 or MD-88 to LGA. I should be able to upgrade the whole way most of the time. Total time is about 3 hours (in flight).
1b) LIT-LGA on AA. Two options here: LIT-DFW on an RJ, then an MD-80 to LGA. I upgrade for 3 hour of a 4 hour (flying) trip. Or I could go LIT-ORD on an RJ, then ORD-LGA on an MD-80 or possibly a 757. Upgraded for 2 hours of a 3.5 hour (flying) trip.

2a) LIT-LAX on DL. MD-88 to ATL, then possibly a widebody to the coast. Possibility of upgrade for 100% of the flight. But I have to go about 2.5 hours the "wrong way" (LIT-ATL, then however much time it takes to get back to the airspace above LIT). Total flying time is probably 5.5-6 hours(?)
2b) LIT-LAX on AA. RJ to DFW, then probably a 757 to the coast. Upgraded for about 2.5 hours of a 3.5-4 hour (flying) trip.

3a) LIT-CMH on DL. Either an RJ to CVG then another RJ to CMH (possible big plane), or an MD-88 to ATL then a 757, 737 or MD-88 to CMH. Upgrade potential costs me about 1.5-2 hours.
3b) LIT-CMH on AA. RJ to DFW, then either RJ or MD to CMH. Possible upgrade for 2 hours of 3 total. RJ to ORD, then RJ or MD to CMH. Possible upgrade for <1 hour of 2.5 total.

After all that, I'm scratching my head on why I don't live somewhere else (topic for another thread, I guess).