If students are allowed to pick what subject they want it can help them in the career they want. For example, if a child wanted to become a homeroom teacher they could choose the subjects of social studies, math, science, and writing. And if they wanted to become a art teacher the could the subject art and other subjects.
The problem with the question is simply part of elementary school children: they're not mature enough to make the choice. Given the choice, most third-grade children would probably choose finger-painting over math because it's more fun. And the proponents of this would probably say "let them have fun, what kind of monster doesn't want kids to have fun?" Then they grow up and blame someone else for their inability to not do math.
There is far too much politicization of our schools already. Elementary school should be about establishing a solid foundation; the fundamentals/elementals (hence the name "elementary"). One of the biggest issues in our education system is that we've moved children away from "the three R's" and into subjective matter meant to make them more pliable to political indoctrination. Teach kids the basic, non-political lessons now and when they grow up they can form their own opinions.