Okay, so I found a flight cheaper than a regular saver award: DFW-MSY roundtrip on Wednesdays in September. Dynamic price 22K miles + $5. Saver price 25K miles + $5. Cash price $86 each way, $186 all-in.
22K miles to save $181 is about 0.8 cents/mile return. That's pretty poor. Plus compared to the regular award, I lose the ability to put it on hold, and to make free changes: that's significant.
Unlike many here, I do sometimes use miles for domestic travel, and since flights from DFW are all short, it's rarely worth it to burn the extra miles for F. But my threshold is high, tickets have to be expensive before I'll use miles instead of buying.
If this ratio of miles to cash cost is typical, it's hard to see when this would ever be worthwhile.