Personally I don't think preschool is necessary but then I am planning on homeschooling so I'm in a different situation. I think if your child will be attending kindergarten then preschool may be a good introduction to school. It really depends on the preschool and teacher as to whether or not it is truly beneficial to your child.
Here's a previous thread that may interest you as it had all different opinions about preschool
Here's a previous thread that may interest you as it had all different opinions about preschool