Tinker I agree with you as far as my personal game goes. It is easier for me to put the shorter irons close just because they don't travel as far on the wrong line. And I guess that principle still holds true with single length since there still are distance gaps due to loft. I have heard and seen other golfers though that, for some reason, they hit one short iron better than any other in their bag, and I guess if that happens to be close to 7 iron length then it makes sense for them to make most of their clubs that length. I guess another argument would be that, if you took just one club and practiced with that club continuously, you would hit that club better and more consistently than any of your other clubs. So if many of your clubs are the same length, you are practicing the same swing no matter which you are using - that makes sense to me.
My own personal experience with a progressive length set is that when my swing is good, I hit them all pretty well on full shots. When my swing is bad, most shots I hit will be bad regardless of the length. The problem is that I don't practice enough to be consistent and to keep my body moving the same day-to-day. The single length idea never appealed to me for that reason. I would still have to make myself practice to get good đ.