Arguments against it seem compelling to me:
1. The American dream is achievement through hard work. But taxation of capital gains privileges ownership over labor. That's just un-American.
2. This preferential taxation creates an anti-productive industry dedicated to re-casting ordinary income as capital gains, leading to misallocation of resources.
3. Many states including California tax capital gains at the same rate as ordinary income; CA is not exactly suffering from lack of investment income.
4. Optics are terrible: I and many other ordinary Americans pay a higher average tax rate than the very wealthy, including ex-presidential candidate Mitt Romney and in all likelihood President Trump.