Thanks, but I still don't get it. In a community property state, personal expenses paid from commmunity funds (it's not who's sick, it's who pays) are split 50-50 like the income. So it would seem all they gain is a higher tax bracket and lots of limitations, including that on the child tax credit.

There would have to be an exception to the general rules of community property (like medical expenses paid from separate funds) to have a benefit to MFS. And, as you note, that benefit would have to be big enough to offset any limitations that apply, including the possibility of a higher tax bracket on one of the separate returns.