Regarding the discussion on accounting for "normal expenses" that you don't have, I don't think such things should be included in a person's spending number (as they're not spending money on it). However, I think anytime the number's being used for something that isn't "just personal" (i.e. your case study, your information, etc), a "caveat" should be included. "I spend $2,000/year, however I live with my rich parents who provide free housing, food, utilities, cars, boats, and a private jet as well as access to their 12 houses around the world" is a far cry from "I spend $2,000/year and live wonderfully" which doesn't mention the parents supporting them in an extravagant lifestyle.
For instance, my plan is to allot ~$60k/year for spending in retirement, with a paid off house in an average cost of living area, and with the VA covering my medical expenses. Someone spending that including a mortgage ($12k/year for me currently) and covering their own health insurance and medical bills (upwards of $10-15k for an individual in my situation without subsidies) would have a significantly different "personal spending" allowance ($60k for me vs $35-40k available for them after the bills I won't have).
As of this morning, almost 40% of respondents either identify as Mustachian or kinda-Mustachian. I think that's pretty good honestly. I think if I had a lower income I'd probably shoot for being kinda-Mustachian myself. At my income, as a single guy with no kids, however, I'm content to just save enough to retire in my 40's and maintain my current lifestyle (well, likely make my lifestyle a little better as I won't be going to work so I'll definitely enjoy myself more lol). I think it would be interesting to see how income plays a role in people's choices to "fit" into each category, though I'm sure a poll or something could be set up to determine that I'm not sure how it would be done and don't have the interest in setting it up. I'd hypothesis that many people with lower incomes would be more likely to adopt more of the advice of MMM than those with higher incomes, though I'm sure there are some in every income category that would defy that.