My connecting flight earned me 3 extra flight miles!
 

Summary:
DFW-DTW (Detroit) is 987 miles
DTW-STL is 440 miles and STL-DFW is 550 miles giving a total of 990 flight miles
----------------------------------------
About a week ago I paid the extra dollars to fly DFW-DTW direct.
For the return leg I booked the connecting flight which was less expensive.
When I arrived at the gates I saw a direct flight DTW-DFW almost ready to close doors. Gate agent says "no way" I cannot fly home direct, I must take connecting flight. I realised that they realised that I have a connecting and cheaper ticket.
So I flew DTW-STL and then STL-DFW thinking that I'd pocket some extra miles. My little mileage run - just like the other people on FT.
Yes, my mileage run gave me 3 extra flight miles. Including the bonuses and minimum of 500 per segment I bagged an extra 79 miles from the connecting flight. At about .02 per mile my gain was....
Mark, Dallas

actually more, you will get 1050 base miles since the min. miles per flight is 500.

I flew DL one time from BOS to DFW via CVG.

BOS CVG 752 miles
CVG DFW 811 miles
Total 1563, with no short segment extra.

BOS DFW nonstop is 1561 miles.

Geographically, does this make sense?

Do these airports really almost for a 'straight' line?

Sure does:

http://makeashorterlink.com/?S445521A

Yes, and you can do the same thing with other warped "triangles":
DFW-SJU = 2165 miles, but DFW-MIA=1121 and MIA-SJU=1045, for a total of 2166... a whopping 1 extra mile for connecting in MIA.

You should have connected through ORD.

DTW-ORD 500
ORD-DFW 802

Total 1302

I always prefer the "backwards" connections:

IE: LAX-SFO-ORD

or

ORD-CVG-DFW