Keep in mind no one has ever lost money in a US regulated futures account. In the rare case of a broker failure you may have to wait until the dust settles to get all your funds; so for some, there might be an opportunity cost.
The Corzine case should be enough to scare anyone from keeping large sums collecting in a futures account. Reading the article below doesn't convince me that this was all on the up and up. As the saying goes, Corzine probably knew where too many bodies were buried so "the lost funds" miraculously became unlost.
The smart thing to do with a low margin futures brokers is to sweep your profits out on a regular basis.
http://fortune.com/2013/11/15/how-mf-globals-missing-1-5-billion-was-lost-and-found/