My question was to anyone who wants to answer it....
The "marginal vs. effective" analysis is one of those things that seems self-evident at first glance, but in reality is not correct.
There have been several threads here and at Bogleheads discussing this. If the wiki referenced above doesn't explain it well enough, I can probably dig up links to some of those threads.
I think it is correct
IF you expect tIRA withdrawals or trad to Roth conversions to be the your sole taxable income in retirement. In that situation (which I think many people fall in), "marginal vs. effective" is correct and accurate.
For example, my personal situation is that I'm a relatively high-earner right now (HH income at 80th percentile nationally), and I hope to retire by 40-45 and spend several years using my tax-free space to do Roth conversions while living on savings in taxable accounts.
My calculations are that I'll have the following tax-free income limits (depending on my children's ages when I retire and assuming that the basics of the standard deduction, exemptions and Child Tax Credit remain the same):
-3 years of ~$50k tax-free while I have two kids under 17
-2 years of ~$40k tax-free while I have one kid under 17 and both kids as dependents
-2 years of ~$30k tax-free while both are over 17 but still dependents
-2 years of ~$25k tax-free while one is still a dependent
-??? years of ~$20k tax-free with no dependents other than my husband and myself
That's $340k worth of completely tax-free conversions over 9 years, while also paying 0% LTCG on the taxable withdrawals. If needed, I could bump my withdrawals up to the top of the 10% bracket, gaining an extra $10k-$20k/year, plus there may be college tax credits that I'm not considering.
Obviously, this won't work if your situation is different, and there's a clear difference between having dependents and not from a tax perspective. PFT's situation is significantly different without a way to get this kind of tax-free conversion space (because he's using it on the pensions), but I think I'm pretty safe in saying that pensions are the exception rather than the rule these days.