If you're applying for an FHA or VA loan, sometimes mortgage bankers are more adept at some of the intricacies involved than a mortgage broker. For example, the tract you are buying in may not be "approved" by FHA or VA. Mortgage bankers often have more clout in getting it approved than would a small mortgage broker.
If your home loan is declined for some reason, many mortgage bankers allow their loan officers to broker the loan to another institution. However, because your loan officer is so used to promoting his own company's product, he often loses track of the "niches" offered by certain wholesale lenders.