I'll let jcl answer this more professionally, here is just my take on reinvesting profits:

First of all, I don't know which timeframe these calculations have been made for. If you trade for an extremely long time with reinvesting a fixed percentage of your profits, a margin call will occur sooner or later, I hope that's clear.
Now it's the question what an 'extremely long time' is.
As the Drawdown shown in Zorro Backtests is normalized over three years, you might be fine over that period.
However, if you trade for a very long time with all your accumulated gains at risk at any given time, the probability of you losing everything is quite high. The goal of reinvesting profits with the squareroot-thing is to keep the probability of a marging call independent of the trading period, hence it remains constant.

Here is a comparison:
The important part about reinvesting profits is that when you get a margin call, you lose your start capital and everything you have gained so far. So you probably wasted your last 5 years looking at a rising equity-curve and now its all gone.
The probability of a margin call when you reinvest a fixed percentage of your account balance is the same as when you don't reinvest but withdraw your gains regularly.
In the latter scenario, however, when a margin call happens, the odds are good that you have already withdrawn huge profits that exceed your initial capital. In the first case you lose everything.
You can see the square-root thing as building up a safety net which protects your hard-earned profits.

Hope this helps a bit,

blaubär