The reason I think EVERYBODY should pay taxes is pretty simple. First, the poor is a group that is not even noted for voting. My thought has always been that if you have no skin in the game then why bother? I used to have a couple businesses that hired a lot of the poor. Every election I got them all together and explained a simple fact; "if you are poor you are going to get screwed. Politicians know which groups vote and those are the groups,that get paid attention to". I never told them how to vote only that they should. I failed miserably.
Capital gains - you don't pay the same taxes as on normal income, you pay less. I know the logic but disagree. Income remains income. There are lots of those kinds of things in our tax code. Hell, the last so called "tax break" have some interesting examples.
The logic behind depreciation is simple. Its a cost of doing business and our tax code allows us to take these costs off before arriving at how much we owe. Over the years this has moved around, ie. how fast can you depreciate, is depreciation considered the same as buying a truck used in business, etc.
I can't go into all this stuff in detail but its just taxes and the 'deals' that help the businessman, investor, etc. The problem with these things (deductions) is that they can be pesky. It really boils down to whether you pay taxes on the net income or the gross income. The real problem is the simple fact is that we now have a system which is at least as complex as any religious book and actually contributes to our economy (accountants, bookeepers, attorneys, etc). I wonder what would happen if we had a system where business had standard deductions based on business category. Lobbyists would have a field day I fear.
Microsecond stock trades come under the heading of "Because they can". As long as we have markets its gonna happen. That's one reason I want them taxed. We could call it "Microtax" <g>