@Lawper at that point you can look at the effect of competition. Why look at the guy with a 26 and then a 35 when you can take the guy with a first-time 34 instead? There are plenty of those to go around.
Also although the MCAT is used as a proxy for standardized test taking ability along with a small to medium amount of content mastery, the fact that MCAT retakers can be highly successful in admissions means that not nearly as much emphasis is placed on it as step 1 anyway. So it is a proxy, but it's not a perfect analogue (which I think is obvious to everyone, as well as not the intended purpose of the test) and adcoms know this.
Going back to the original point (or at least one more upstream), this is why complaining about how much the MCAT can change your chances for med school admissions is pointless when you're entering a profession where each successive step is going to be even more high stakes.
It's important to note that the guy who went from a 26 to a 35 actually got a 35. Is it as good as a 35 by itself? Clearly not, but it's situational. Adcoms clearly aren't one-dimensional to blindly choose a 34 guy just because of one attempt, since otherwise:
1. Retakers wouldn't get into medical school in the first place
2. AAMC would enforce a one-shot MCAT deal (and no school/adcom would have a retake policy)
MCAT is used as a proxy for Step 1/boards analysis only because of its standardized nature, which we can agree. And yes, each step gets harder than the last, but at least there is a consistent motivation and support system along the way, unlike the ambiguity surrounding the MCAT.
This whole discussion definitely plays some role in a) why there are ADCOMs who aren't fans of re-taking MCAT scores that are sufficient as is. b) why so many schools will average multiple MCAT scores c) why as gyngyn said he knows just as many individual evaluators who put more weight in the poorer score as those who put more weight in the higher score for applicants with more than one MCAT score.
As a side note it's worth noting there are also a fair number of RD's and others at the type of level and ranking who will still tell you they wish that Step 1 was merely a pass/fail exam like its original intention was and don't support how Step 1 is used in residency admission purposes.
Adcoms can play all the numbers games and do a bunch of advanced statistical analyses for all I care, since they also arbitrarily assign random weights to ECs and do some calculations. The admissions process is chaotic and so sensitive to errors, human biases, connections, impressions etc. This is why forecasting the admissions even for the best applicants is a fool's errands. And what each adcom say is best used as a guide
I consistently agree that taking the MCAT once and doing it well is essential. I also agree that retaking a good score is discouraged due to increased risk of doing the same or worse in the second attempt. What is going too far is in the small cases is the random stigma against people who do retake good scores and score even higher. Of course, adcoms can intellectualize all they want since it's their call, but the results of these cases say otherwise.
The reason Step 1 is a one shot deal is because that's the way medicine and the professional world is. You get one shot. No re-dos. That's the standard by which you are judged. Nobody craves for medical students who can succeed after failing or not achieving a standard. That's just not good enough in that field.
The timing issue about Step 1 isn't the key in terms of why you can only take it once, just as there is no other part of med school you are magically allowed to re-do if you don't get the score you want unless you fail. The school investing time in you isn't the reason behind Step 1 being a one shot deal. Everything counts at that level, and you get one shot to do it the way you need to.
And that schools are literally investing time and resources for Step 1 prep. Why are you ignoring that factor? Retaking Step 1 means that schools waste even more time and resources to make sure you do well and not look bad.
There is no reason for MCAT to be a one-shot deal because no one is investing resources into your doing well. No one cares. Only you do.