The downside is that such memberships are fairly expensively. For many teachers, belonging to a union is a necessity because of the benefits that one receives. In many cases, teachers receive discounts, and these discounts usually tend to be fairly generous. Sometimes, the savings offset the cost of the membership. For me, the cost of term life insurance is significantly cheaper through my teacher union than it is through the insurance programs that my employer offers.
Many people just join unions because many teacher unions offer free legal representation and malpractice insurance. The hope is that you, as a teacher, will never need legal representation or malpractice insurance, but it is better to be prepared than to find yourself in a bind.