Another way to look at the value of miles is when considering bump (DL Travel) vouchers. Today they "reward" you with $200.00 to give up your seat.

If you consider that it takes 25,000 miles for a free coach roundtrip in the US is 0.02 * 25,000 = $500.00

Therefore, if you just want to fly a cheapy coach roundtrip between say, Fort Walton Beach and Atlanta (< $200.00), you could accept the bump, use the voucher, and still have a few bucks left over. Alot of people will also tell you they feel better about burning FF miles only when taking longer DL mainline jet trips, so you are getting more bang for the buck. Short haul commuter flights are really a waste to burn miles on if you can help it.

Also note, that when travelling on an award ticket (attained via cashing in miles), you don't get skymiles for the trip - whereas the voucher does earn miles.