The dilemma you raise is a very personal decision for most people (especially those here, who seem to care more about miles than the general public) -- dependent on individual flying patterns, personal finances, and priorities (e.g. do you care more about optimizing the value of your miles, saving for emergencies, or get extraordinary satisfaction by just watching them grow?)

Having said all that, I personally wouldn't dream of using miles to cover a $120 ticket, even with the new reduced mileage awards. My hurdle rate for a 15,000 mile ticket, taking into account the opportunity cost of forgoing current double miles promotions, would be around $450 (or around $.03 per mile), depending on the destination. But, as I said initially, everyone has their own opinions on this subject.