> The other scenario I could see with filing separately is if our combined income bumps us up into a new tax bracket (which I can't imagine happens often).

Just to provide some clarity and save research, that can't possibly happen unless you make $647,850 or more combined.

If you look at tax brackets, you will notice the joint numbers are exactly double that of Single (and Single brackets are the exact same as Married Filing Separate). Same holds true for the standard deduction. So if you make $50,000 taxable income (22% single) and your spouse makes $50,000 (22% again Single) then your joint bracket is $100,000 (22%) and it's impossible to ever go above that. Your marginal rate for joint can be equal to your separate Single brackets (if you make similar incomes) but it cannot ever go above that, at least at <$648k.

The opposite can happen, which is why people with disparate incomes benefit from joint. Say you made $100,000 and your spouse makes $0. You would be in the 24% bracket and your spouse would be in 0% if you did Single. But if you file joint, you're back down in 22% land. So it's like you can "move" your income off your stack onto your spouse's. So you move your 24% portion and some of your 22% portion down to your spouse's 10%, 12%, and 22%.