A will would do the same, so I don’t see that as an issue. Nobody is entitled to the money of another at death.
It would allow a gay spouse to get the benefit of any marriage adjustments in the estate taxes — spouses can inherit with no tax in most cases. But I can’t say I’m all that upset about that, because all of us should be able to inherit without tax (and most of us will, at least federally, with the 5 million deduction).
That is why they chose this case — it is the one most likely to invoke some level of sympathy even among opponents of SSM. Once this case grants the right, then they get it all, like forcing christian business to provide spouse benefits for same sex couples.
The proper remedy IMHO would be to provide a means by which states can recognize civil unions between arbitrary adult couples who could not legitimately be married. Membership in a civil union would be orthogonal to the propriety of sexual relations either between the union's members, or between a member of the union and someone else. Consequently, such a union would not have to be restricted to persons who are not blood relatives; two widowed siblings, for example, might wish to form a union to care for the children they bred with their late spouses. The siblings would not be considered spouses, but each would be co-guardian of the other's children, and each would have survivorship rights in the other's estate.
Oddly, however, civil union laws often expressly forbid siblings from forming unions, even though since civil unions are supposedly "not about sex" incest should be a non-factor.