Quote From Dunham
You're quantifying experience without considering the vast amount of variables, and using a hard science as your example without considering that there are plenty of other disciplines in which the PhD experience is vastly different. The argument is not easily transferred.
2 more years experience isn't necessarily 'more experience' if you have a crap go of it. In fact, those two years could potentially be a waste depending on how it pans out.
You say it's objective, but then contradict yourself by saying it's highly individual. Would that not then suggest that the experience is not quantifiably objective, but rather, subjective? Wouldn't it also depend highly on the discipline, the school, the graduate program, the supervisor(s), access to resources and equipment, access to funding, access to quality mentorship, access to support systems, the individual's personal life and whether or not they have disabilities, experience systematic discrimination, have family commitments or family breakdowns, have health issues?
All of those things will impact how a person performs in their program and the quality of experience gained (and really, it's quality over quantity). 2 more years might *seem* like more, but this is only a surface examination. It doesn't say anything about the quality, or what the person can do/has done.
If you take a bachelor degree as an example, many students get 3 and 4 year bachelor degrees. But not all of them will have the same quality of education or 'experience.' It might be perceived as such, and yes students will come out with more experience, but more isn't better, it has no meaning if you don't consider what this experience actually looks like.
I might have a PhD but that doesn't say that much about the quality of my work. My research output, my ability to attract grants, my projects, my teaching, all of that says much more than a very basic quantifiable measure of how long I went to school, which is what you are basing your argument on.