good post.
i agree that sometimes to change things up is good. a new stimulus to get things rolling out of a possibly stagnant situation is often the key.
also i've noticed, in years of coaching and observing, that to ask a slower runner to knock out several 18's and a 20 or two will more often than not leave the runners more fatigued than necessary, and with a greater chance for injury. frequently, they can still finish the marathon, but the last 8-10k "death march" is more likely to occur when they begin the marathon over-fatigued, or under-rested.
perhaps we have attached a mythological importance to the twenty miler. science has taught us that the body depletes energy stores at about the time it takes some people to run twenty miles, and so there is some value to that idea. but there are ways to teach the body to deal with that, and i think that some training programs--like the hanson's, et al, address that.
if we can wrap our minds around an idea, likely it will work for us, whatever that idea is. belief in what you are doing--in your training program--is one thing that ought to be considered when entering the marathon world. i see too many people bouncing from program to program, looking for the quick fix, when they might be better off paying attention to the bigger picture.
the hanson's have a sound program, undeniably. is it for everyone? probably not. but theirs is one that has history, science and empirical evidence behind it, and is similar to what many other coaches of greater visibility and acclaim have done. so i think it is unfair to knock it based simply on one element of the program.