One thing to consider is how having a large amount of stocks allows you to borrow on margin and move other people's money around in ways the less wealthy cannot. I wonder if the aim here is to tax the power, not necessarily the wealth.
I'm all for taxing people whenever they benefit from their wealth. Stocks sitting in an account somewhere? Those aren't benefitting the owner. Stocks used to borrow money? Now they become a tangible benefit, and we can figure out how much to tax that event.