Quote from jimrockford:
I forgot to mention certain things. Like I said, if you are going to hold positions, in addition to your cash, then you should liquidate enough EFPs to cover those additional positions, so that you will not be charged margin interest on the additional positions. Margin interest charged by a broker will generally always be greater than any amount of interest you can earn on the same money, via an EFP or any other vehicle.
BUT there is an important exception. If your additional position is a short or long of a stock which has a corresponding SSF, THEN you can use an EFP to finance your additional long or short position, and be in a better position than you would otherwise.
If you are going long a stock for which there is an SSF, then you need not liquidate your EFPs to free up cash. You can instead first purchase the stock, and then buy EFPs. The result will replace your long stock with long SSFs, until the SSFs expire, at which time your long stock will be replaced. The amount of money you lose on the EFP spread, between shorting your stock at a lower price and buying the SSF at a higher price, will be less than the amount any broker would have charged you for a margin loan to finance the stock purchase. So you can use the EFP to avoid paying margin interest to your broker, and instead pay a substantially lower equivalent interest rate using EFPs as a financing mechanism instead of a margin loan extended by your broker.