For example, if I want to jointly own a home (and only a home with no other possessions jointly shared) with someone, we would not receive a total $500k capital gains exemption on the sale of the home after 5+ years of ownership. Only married people filing taxes jointly get this right. Instead, me and the other person would only be allowed a $250k exemption. How can such a situation possibly be fair? The correct abstraction would have been to allow up to $250k capital gains exemption for each individual on the deed. [0]
At the end of the day, we should be designing laws the way we design software. Strong separation of concerns should be a design goal when drafting legislation.
[0] http://charliedunn.com/_Sellers_Tips/Capital_Gains_Exclusion...