the senior guys in the union ran the numbers, the argument has always been that trip value was "guaranteed" money. Therefore you know what you will make in a month. You don't lose money in the winter, the spread is in slightly in the companies favor (with a delay, you will still make the same, or work for free). During the summer, the spread is in the pilots favor, you will make more and fly less. Due to the yearly adjustment of trip values, those pilots that try to hedge the trip values (by hauling butt) during the summer eventually give it back to the company when they next evaluation is done.

bottom line, all trip values do is guarantee a fixed amount. When you get to 15+ years and can live on 75 hours alone, trip values is what you will desire. Least amount of hours for a guaranteed paycheck.