I called CME risk-management about this to clarify. The rep told me (paraphrasing)..."The requirement to get into a future contract is the maintenance margin of that contract. If it is a non-hedge/non-member account, it's the initial, which is 10% more. There is no such thing as DT margin at the CME."
This leads me to believe that a broker offering a $500 DT margin for a contract, say ES, which is $5,225 for the initial, is requiring $500 of the speculator's account money, and the non-hedge/non-member FCM is putting up $4,725 of money from their own "accounts" to cover the position.
So a broker who is helping to cover a traders' costs to initiate a position (hence the term "initial margin"), is taking on risks of their own.
I don't understand the FCM business, and don't shoot the messenger. This is what the rep said in a nutshell.
The summary that we all forget is here--->
http://www.cmegroup.com/clearing/cme-clearing-overview/performance-bonds.html
This leads me to a conclusion. Always trade only what you can to meet the initial per contract, regardless of what your broker says you can or cannot do, if neither you nor your broker wish to suffer any major surprises in the rare "flash-crash" scenario.
WITH THAT SAID however, that does not mean that low DT margins are not profitable for any one particular broker.
That business model must be working, or else it would not be used at all by anyone. Makes sense, yes?