On course length:
Only by using your Jones Counter, and following the strict calibration methods required for it, have I gained a deeper appreciation for the accuracy (and precision) of certified courses. The method takes time, and a lot of care, but in the end is pretty rewarding. Thank you for it. Furthermore, having used your counter makes me skeptical about the true length of courses that don't take such measures.
My guess is there's lazy-streak in race directors, who think that a car or the latest in-vogue gmap-pedometer measured courses "will do". From a certain standpoint, runners are becoming less savvy consumers, and allowing races to perpetuate these shenanigans. Also, it is not right that USATF breezily sanctions races without the courses being certified, too. Kind of a dis-service to it's $50-a-head paying members.
On the inevitable "then versus now" performances argument:
I would guess that there are more races of most any distance in the US now than at any earlier point in history. If there were the same number of "fast" racers in the country, but spread out among more races, and diluted with a lot of neophyte runners at the back, then of course you are going to see much higher percentiles for a given performance. I think you have to look at omnibus results across all races in a given year to get an idea of where we're at, not Boston 1981 compared to Boston 2006. More like Boston 1981 compared to Boston + NYC + Chicago + CIM + etc. 2006.
The draw to many of these races, some costing upwards of $100 a pop, is not the same now as it was in the nominal-cost days of the 70's and 80s. I think that must influence the decision of today's runners in their 20s and early 30s, whether to compete at all. I've had good, competent training groups break up in the mad dash for a couple of hundred dollars worth of comped race entry fees and some free gear. Needless to say, this kind of desertion has not been necessarily helpful to make this 20s to early 30s 2:30 marathoner any faster!
One suggestion: Institute a progressive race entry-fee system, subsidizing the faster runners, and supported by documentation of earlier bona fide race results. I guarantee you would see a lot more progress, because then athletes would receive small, but constant, incentives to improve. I think this would cost very little, but with pretty substantial returns. I think this would also work to return the focus of races to true competition, and not just participation. It might even make potentially outstanding runners hungry once again.