CMC markets offer attractive brokerage rates ($9.90 minimum or 0.1%) making commsec, nabtrade, and etrade for example overly expensive. For example $15000 traded with cmc will cost you $15 rather than 29.95 with commsec and nabtrade making cmc the broker of choice for certain.
But they give you pretty well nothing in interest whilst you have money parked in their account waiting to trade. I am unwilling to trade with cmc amounts under $10000 as I do not wish to give up approx. $300-$400 in interest per year for this $10000 amount. I have my money parked in a nabtrade account and pay $15 with nabtrade.
CMC markets use a Bankwest account that pays you next to nothing; they could have used or allowed a linkage to the Bankwest Telenet saver account which pays decent interest. They won't allow any more than one linked account which of course has to be your account that you can withdrawl from (the telenet account will only allow one linkage and no cash withdrawls also).
All designed to take your interest: a component of cfd providers profitability is the interest that they get from your money that they take instead of you getting it.