Quote from darkhorse:
Well, for one, portfolio margin will give you increased leverage only under certain circumstances - and they may not be the circumstances that are ideal for your strategy. Keep in mind too that, under certain concentrated position circumstances, portfolio margin will give you less leverage than Reg T.
If you have allocator leverage, on the other hand, you can take your leverage up or down as you choose.
So think of portfolio margin leverage as coming with all kinds of caveats - and for directional traders, the concentrated position caveat is a HUGE one.
Let's say, for example, that a major, major macro event has just taken place. A discretionary trader is already up 20% for the year, having brilliantly played the secular bear decline.
Now, in the flash of an instant, this trader sees that the entire game has changed... that the time has come to go "long the world," so to speak -- risking 500 basis points, or a quarter of his accumulated profits, to do so -- and he is able to do this with a very tight risk point, say in S&P futures, because of the epic inflection point that has just crystallized, allowing him to put on a HUGE amount of upside leverage in a very concentrated space.
Our hero trader tries to go long S&P futures, all above considerations taken, with his portfolio margin account.
The portfolio margin engine says "sorry, no dice - that's too much concentrated risk." One of the biggest opportunities of the year, maybe the decade, to make a sick killing has been cut short. Our hero trader makes a lot of money, but not NEARLY as much as he could have made, had he not been reined in by a robot.
With allocator leverage, there would be no such automated restriction on concentrated positioning. Allocator leverage is thus not subject to one of the biggest flaws of treating portfolio margin as a source of leverage - the fact that concentrated positioning is not always a negative, depending on planned risk, situational dynamics, and other factors.
And the same can apply in complex ways to more advanced market neutral type strategies, though the details are too hairy to come up with a quick example here...
Bottom line being, portfolio margin leverage is not "true" leverage in that it is not "always available" leverage. It is subject to a pre-determined robotic formula, the calculations of which do not alway properly account for situational dynamics.