The problem of spouses beating spouses into voting differently (or parapalegics having their votes manipulated) is not on the same level of low voter turnout. Mailing in WA ensures much higher participation than the possible outlier manipulations that might happen (the number of calls about some ex-spouse having manipulated spousal votes is in the number of 0/yr for King County, WA). You get a tracking stub, by which you could contact the election board, if that was really an issue and...it's not.