I think you are remembering incorrectly ... like the DL agent told you, they would sell you the last seat on the plane if you called a certain number of hours ahead (24 or 48 I think), but not just give you an award for the standard mileage amount.

Note that any member of SkyMiles can redeem an award for any open coach seat just by using double the number of miles normally required for a SkyChoice Award (I think that's the right name, without going to look it up).