Quote from timcar:
The obvious answer is to increase rate of return. By:
1. Holding more shares in a potential profitable position.
2. Being able to diversify into other positions.
Nothing some of us traders hate more is having to sell and buy into another "hopefully" better profitable position. The generating of two commissions in order to complete these trades does upset some traders.
So, the additional capital being borrowed by trader will prevent the two commission trade problem and some traders are inclined to chance/risk the margin call.