Hmmm, that is one cheap brokerage. How long have you been using them?
The caveat is the margin call. Stock prices drop, then you get a margin call. If you don't have extra money to cover it, you are ruined.
A lot of people got ruined this way during the great depression...
As for credit cards, it's still cheap use of capital without the risk of margin call. I can get 0% for 1 year, sometimes up to 18 months for a 3-5% balance transfer fee.
On $25,000, that $1000 in fees. I make $12,000 a year off that, so it's not expensive at all. Plus I don't have to worry about getting ruined and losing everything with a margin call. I'm not in a high tax bracket, so I'm not too worried about taxes.