As a broker dealer or market maker, you do get better margin requirements than customers. However, as a broker dealer or market maker there are a few disadvantages.
First, you CANNOT use the system to enter orders on some exchanges (in other words, you will need a floor trader to enter your order for you).
Second, you will have to yield to customer orders at the same price even if you have time priority.
Third, the published quotes may not be honored. They are for customers only and you will probably get faded a lot.