There is a reason the wrenches in wrench sets come in 2" increments. If your HANDS aren't sensitive enough to perceive the changes in leverage of less than 2"....
I imagine that market forces, rather than sensitivity, dictate the increments. Besides, I can use an 8" wrench pretty much the same way I can use a 6" wrench in most situations, plus, I can use the 8" wrench pretty much the way I would use any wrench between 6" and 8".
You're also ignoring the large amount of anecdotal evidence that increments of a few mm make significant differences. Not long ago I raised my seat 4-6 mm and stopped experiencing knee pain. Not at all definitive, but there are probably 1000s of similar experiences documented just in BF archives - too much testimony to ignore, IMO, especially when combined with advice from coaches, fitters, and riders.
The fact that small increments may not work for you (and that remains to be proven) does not mean they won't work for others, especially if they're riding at their margins.