Two observations that came immediately to mind while reading this post:
1) Accountants and lawyers are people with jobs. Fire them and replace them with non-accountant, non-lawyer jobs, and all you've done is shuffle the jobs around.
Yes, you can argue that every dollar spent on non-administrative expenses can be invested into productivity, but that leads me to my second observation:
2) Small businesses just don't spend that much on administrative overhead.
I've worked at far more small businesses (<20 employees) than large ones, and all of them -- every single one -- has out-sourced the accounting expenses to such a degree that I doubt there's a single full-time equivalent lost in the shuffle. Legal expenses can be more variable, but typically not because of government overhead.
When I hear this argument coming from extremely rich people, my natural instinct is to assume that they're using the populist vision of the "struggling small business owner" to promote policies that would benefit their personal bottom line. Quite honestly, this looks like more of the same. Mark Cuban has a lot more to gain from cuts in the corporate tax rate than does your average small business owner -- yet he'll never employ enough people to make up the difference.