I've read elsewhere that DNA polymerase is actually a series of enzyme subunits working together as a complex rather than a single enzyme. This makes sense when you think about it though. Considering that an enzyme typically corresponds with one reaction, there must be many involved to achieve DNA replication. DNA is a complex molecule and thus it would require a number of reactions and organization to achieve true replication.
Another function of helicase is to prevent the DNA in the process of replication from getting two wrapped up. Similar to how a rubber band is changed when both ends are twisted. When DNA is put into such a conformation it is unable to be copied due to the torsional strain on the molecule. Thus, it is critical that helicase performs this action as well as simultaneously working to separate the two strands of DNA.